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THE THEORIES 

OF 

CHEMISTRY 



By 

EDGAR F. SMITH 

ij 

Blanchard Professor of Chemistry 
University of Pennsylvania 



PHILADELPHIA 

THE JOHN C. WINSTON CO. 

1913 



Q31453 
S5 



Copyright, 1913, 

BY 

Edgar P. Smith. 






PREFACE 

This voltime represents a desire on the part of the 
writer to place in the hands of his students a concise 
account of the development of the numerous theories of 
chemistry. It is his purpose to use it as a text-book, to 
be supplemented by lectures. And it is further hoped 
that the material presented may serve as a basis for 
more extended study. No claim is made for originality. 
It is an old, old story — ^many times told — gathered from 
innumerable and even forgotten sources, to which most 
cordial acknowledgments are here made for all bor- 
rowed facts and statements. The writer is likewise 
under many obligations to Miss Sarah P. Miller, Ph.D., 
Girls' High School, Philadelphia, for the skill and patience 
she has shown in condensing hundreds of pages of manu- 
script into the present compact form, and superintending 
the proof-reading and press work with the utmost fidelity 
and care. 



THE THEORIES OF CHEMISTRY. 



The Grecian philosophers were the first persons who 
undertook an explanation of the creation and destruction 
of nature. The chief aim of the pre-Socratic philosophy 
was to account for natural phenomena. Every indi- 
vidual on viewing nature and the phenomena of nature 
regarded that one thing as the original of all other things, 
which to him seemed to be the most remarkable. Thus, 
Thales considered water to be the proto5^e, because he 
knew that without it life was impossible. Pjrthagoras 
said numbers were the cause of all things, for the reason 
that on considering the movements of heavenly bodies 
in accordance with mathematical laws numbers seemed 
to him to be most important and impressed him most 
deeply. Heraclitus said fire was the cause of all things, 
and Anaximander declared it to be matter. In the 
opinion of Empedocles matter consisted of four elements: 
earth, air, fire and water. 

Aristotle maintained that there was an ur-substance 
at the basis of all things. This iu:-substance could not 
be annihilated — it made all things possible. It had 
potentiality, but it was not real or actual. It possessed 
four fundamental properties. It was warm, cold, moist 
and dry. The imion of any two of these properties gave 
rise to one of the four elements of Empedocles. Thus, 
earth was dry plus cold; fire was dry plus warm; water 
was moist plus cold; and air was moist plus warm. 
Differences in the material world were thus ascribed to 
the properties inherent in matter. Aristotle thought 

(3) 



4 THE THEORIES OP CHEMISTRY. 

there was a fifth element, which he called ousia-ether. 
It penetrated everything. 

These ideas of Aristotle and Empedocles are thought 
to have been borrowed from even earlier sources. In 
other words, there were men before them who enter- 
tained similar notions. 



Alchemy added many substances to the Hst of ele- 
ments (earth, air, fire, water and ousia-ether) previously 
mentioned, in order to account for the properties of the 
metals. Geber introduced sulphur and mercury into the 
list. These he considered the basis of the metals. This 
is the first attempt to explain differences in substances 
by assuming a pecuHar composition. Albertus Magnus, 
Roger Bacon, Raymond Lully, Arnold Villanovanus and 
a host of alchemists accepted, in the main, this thought 
of Geber. Basil Valentine thought salt was an element. 
To the alchemist sulphtu* represented the properties of 
alterability, decomposability and combustibility. Mer- 
cury represented metallicity, which was the cause of 
metallic luster and malleability. Salt stood for the 
property that resisted fire. 

During the age of alchemy, the first attempts were 
made to convert the base metals into gold or silver. 
The belief in the ** transformation or transmutation of 
the metals" held a powerful charm and led the men of 
this period to labor incessantly, by every conceivable 
means, in order to attain this end. 



Later Paracelsus declared that the three elements, 
sulphur, mercury and salt, were not only the ingredients 



THE THEORIES OF CHEMISTRY. 5 

of metals but also of all bodies, animal and vegetable 
alike. Paracelsus viewed disease as a disturbance of 
the proper proportion and correct *' make-up" of these 
elements; hence he strove to eliminate the disturbance 
by external means, and in so doing became the founder 
of the school of iatro-chemists. This school made two 
great mistakes. It attempted to account for all life 
processes by chemical means, and it set a too narrow 
limit upon the domain of chemistry. 

The greatest of the iatro-chemists were Paracelsus, 
Libavius and van Helmont, all of whom believed in the 
transmutation of metals, by means of the philosopher's 
stone, and in the elixir vitce. The latter objected to 
the idea of the earth being an element. 



The man who bitterly opposed Aristotle's teachings, 
who placed chemistry upon a scientific basis and who 
freed it from the shackles of "gold-making" and "heal- 
ing" and the old doctrines of the constitution of matter, 
was Robert Boyle (1626-1691). He taught that all 
compotmds in nature were derived from an ur-substance; 
that the many differences existing in these compounds 
were due to the varying properties of the minutest par- 
ticles of this ur-substance, and that the varying forms 
and sizes of these particles, their movements, their posi- 
tions at rest and their positions relative one to the other 
exercised a very marked influence upon the differences 
existing in these bodies or compounds. "These minutest 
particles," said Boyle, "consisted of substances extremely 
difficult to decompose." They entered into and came 
out of the compounds without sustaining any alterations 



6 THE THEORIES OF CHEMISTRY. 

or changes whatsoever; hence these minutest particles 
were elements in the sense of that term as understood 
to-day. According to Boyle, chemical union consisted 
in the attraction of these minutest particles (the cor- 
puscula — ^the constituents) one to the other, and chemical 
decompOvSition occurred when one of the two constituents 
of a compound had a greater attraction, a greater affinity 
for a third than for the one with which it was combined. 

Boyle knew that increase in weight followed com- 
bustion. He had verified this by heating lead in a closed 
retort, but he was so thoroughly imbued with the 
dominant ideas of his time, namely, that increase in 
weight was due to the interpenetration of fire matter 
(union with fire), that it was not possible for him to 
clearly comprehend the nature of the substance govern- 
ing this increase in weight. Mark, that Robert Boyle 
weighed. He called attention in 1662 to facts regarding 
the elasticity of air, and asserted that the volume occu- 
pied by a given mass of gas under different pressures 
is inversely as the pressure. 

Hooke (1635-1665), in England, observed that saltpeter 
when heated gave out something capable of supporting 
combustion. His experiments are recorded in a little 
pamphlet, entitled "Micrographia." 

Mayow, another Englishman (1645-1679), said that 
the spirit of saltpeter and the spiritus nitroaerus of air 
were one and the same, and it was this spirit which com- 
bined with lead and iron when they burned in air. 



These germs of the correct idea of combustion, intro- 
duced by Hooke and Mayow, were doomed to pass into 
oblivion; for a German named Becher (1635-82) was 



THE THEORIES OF CHEMISTRY. 7 

actively promulgating the idea that the phenomena of 
combustion originated in a peculiar volatile escaping 
earth or kind of sulphur. He further taught that metals 
and minerals, in general, were derived from three ele- 
ments: (1) a vitrifiable earth, (2) a volatile earth, and 
(3) an igneous principle. The teachings of Becher repre^ 
sent the first consistent theory of the constitution of com- 
pounds and of chemical action. Becher's third element 
— ^the igneous principle, in the hands of his pupil Stahl 
(1660-1734), became the central point of a very definite 
theory. Stahl named the igneous principle phlogiston, 
meaning combustible. He regarded phlogiston as the 
principle of inflammability — ^the fire matter which existed 
in combination with other bodies. Phlogiston, in the 
mind of Stahl, was a hypothetical element — sl pure fire, 
fixed in all combustible bodies, and was to be distinguished 
from fire in action or in a state of Hberty. He declared 
that all combustible bodies were composed of phlogiston 
and a non-combustible substance. Stahl was blind to 
the fact, noted by English chemists, that a metal after 
its burning had greater weight than before. His hypoth- 
esis or theory, the doctrine of phlogiston, was truly 
a step backward when looked upon in the light of the 
observations of Boyle, Hooke and Mayow, yet it held 
sway from 1720 to 1789, more than half a century. It 
must be remembered, however, that this theory led, 
through experimentation, to the discovery of many 
facts, and that it brought these facts under definite view- 
points. The interpretation of the experimental data 
served to correlate facts for which vague and enigmatical 
explanations had formerly been given. This theory, of 
which so much has been written and so much has 
been said, briefly stated is this: Combustion depends 



8 THE THEORIES OF CHEMISTRY. 

Upon some principle which is present in every sub- 
stance. This principle, identical in all combustible 
substances, escapes during combustion. Phlogiston is 
the principle igneous or the principle combustible. In 
combustion, the phlogiston passes out as a flame, while 
the non-combustible portion of the body remains; hence, 
the term ''dephlogisticated" corresponds to the present 
term ''oxidized" and " phlogisticated " corresponds to 
the term "reduced." A metal lost its phlogiston, was 
"dephlogisticated," when it burned forming a "calx" 
[oxide]; this was a loss of potential energy. When a 
calx of a metal was reduced to the metal by another 
metal or by a combustible body like carbon, it was 
" phlogisticated " — an increase of potential energy. The 
conversion of phosphorus into its pentoxide and of the 
pentoxide back into phosphorus was adduced as evidence 
of the theory of phlogiston. 

It must be granted that in the absence of more accurate 
experiments, the combustion phenomena could thus be 
explained with some degree of probability and apparent 
certaiaty. In this connection it is interesting to learn 
the ideas entertained by chemists, other than Becher 
and Stahl, concerning this igneous principle or phlo- 
giston. Some identified it with light, with flame, with 
coloring matters (particularly prussian blue), with hydro- 
gen, with the principle of levity or of negative weight, 
and the like. 

Stahl, in the enthusiasm of his possession, strongly 
opposed the blending of chemistry with medicine, and 
earnestly warned all of his adherents and followers 
against the pursuit of those ancient alchemical "will-o'- 
the-wisps" (ignes fatui), such as the philosopher's stone, 
the elixir vitse, and the other impossibles. 



THE THEORIES OF CHEMISTRY. 9 

A galaxy of talented men upheld the theory of 
phlogiston. They were Neumann, Pott, Margraff, 
Eller, Black, Hoffman, Boerhaave, Bergmann, Cavendish, 
Priestley and Scheele, each an honest investigator, each 
unconsciously contributing to the ever-increasing fimd 
of experimental proof, which eventually caused the 
downfall of the first chemical theory. Frederick Hoff- 
man, while holding with Stahl that siilphur consisted of 
acid and phlogiston, that combustible bodies contained 
something which might be described as phlogiston; yet, 
thought it very probable that metal calces were pro- 
duced, not by the subtraction of phlogiston, but by the 
combination of the metals with an acid material. That 
was coming very close to the present idea. It is inter- 
esting to observe that he used the term ''with an acid 
material." Shortly after (1778), Lavoisier used the 
term "acidifidng principle" for oxygen, and calcination 
for the union of metals (not only metals but also non- 
metals) with oxygen. And Boerhaave, while not directly 
attacking the doctrine of Stahl, doubted the assumption 
of the existence of a combustible principle and of an 
earthy substance in the metals. So, imperceptibly, 
the entering wedge was surely and definitely fixed, and 
when Priestley and Carl Scheele made oxygen known 
to the world the course of chemical events indeed became 
changed. Yet everyone is aware that there had been 
fore-shadowings of the correct explanation of comibustion 
in Hooke's writings, in the observation of Mayow, and 
in Boyle's knowledge of the increase in weight upon cal- 
cination. These were only fore-shadowings, but there 
had been written that which was nearer the truth than 
any of these observations, namely, the contributions 
of one, Jean Rey, on the increase in weight of tin and lead 



10 THE THEORIES OF CHEMISTRY. 

on calcination; first published at Bazas in 1630 and re- 
printed (1777), together with notes and various letters 
bearing on the subject, at Paris. Rey attributed the 
increase in weight of various metals on calcination, 
to the action of air, anticipating, to some extent, the 
work of Lavoisier, a centttry and a half later. A few 
quotations from the essays of Rey may be of interest. 
His style is quaint and lively, and withal most entertaining. 



''Formal Response to the Question, Why Tin and 

Lead Increase in Weight when they are 

Calcined. 

" Now I have made the preparations, nay, laid the foim- 
dations for my answer to the question of the sieur Brun, 
which is, that having placed two poimds six ounces of 
fine English tin in an iron vessel and heated it strongly 
on an open furnace for the space of six hoiu-s with con- 
tinual agitation and without adding an3rthing to it he 
recovered two pounds thirteen ounces of a white calx, 
which filled him at first with amazement and with a 
desire to know whence the seven oimces of surplus had 
come. And to increase the difficulty, I say that it is 
necessary to enquire not only whence these seven oimces 
have come, but besides them what has replaced the loss 
of weight which occurred necessarily from the increase of 
volume of the tin on its conversion into calx and from 
the loss of the vapours and exhalations which were given 
off. To this question then I respond and sustain proudly, 
resting on the foimdations already laid, that this increase 
in weight comes from the air, which in the vessel has been 
rendered denser, heavier and in some measure adhesive 



THE THEORIES OP CHEMISTRY. 11 

by the vehement and long-continued heat of the furnace, 

which air makes the calx (frequent agitation aiding) 
and becomes attached to the most minute particles, 
not otherwise than water makes heavier sand which 
you throw into it and agitate, by moistening it and ad- 
hering to the smallest of its grains. I fancy there are 
many who would have been alarmed by the sole mention 
of this response if I had given it at the beginning and 
who wiU not willingly receive it being as it were tamed 
and rendered tractable by the evident truth of the pre- 
ceding essays. For those without doubt whose minds 
were preoccupied with the opinion that air was Hght 
would have rushed to oppose it. Why (they would have 
said) does not one extract cold from heat, white from 
black, light from darkness, since so much heaviness is 
extracted from air, a thing inherently light? And those 
who chanced to have bestowed their credence on the 
heaviness of air woiild not have been able to persuade 
themselves that it can ever increase weight, being bal- 
anced in itself. On this accoimt I was constrained to show 
that air had weight, which was recognizable by other 
means than the balance, and that even with the latter 
a portion previously changed and made denser could 
manifest its weight. All this I have done as briefly 
as I foimd possible and without advancing an5rthing 
not strictly germane to this matter, to elucidate which 
at aU points it only remains for me to state and refute 
succinctly some opinions which others have held or might 
hold, and to resolve the objections which might be made 
to my answer. 



12 the theories of chemistry. 

"Whether Lead Increases in Weight as well 
AS Tin. 

"I should have ended were it not that the sieur Brun 
informs me in his letter that having noticed the aug- 
mentation of tin he made the same experiment with lead, 
which he found to diminish by one oimce in the pound, 
which sank him still deeper in doubt, having imagined 
that the same increase should be there foimd as with 
tin from the proximity of their nature and from the iden- 
tity of the process of calcination. But to the experiment 
of the sieur Brun I oppose the experiments of Cardan, 
of Scaliger, and notably of Caesalpinus, already mentioned, 
who says it is worthy of astonishment that lead on cal- 
cination increases in weight by eight to ten pounds to 
the himdred. Shall I leave these persons in strife each 
to sustain his own experiment? I am too pacific; behold 
their reconciliation effected! Some lead is more pure 
than others, either because it comes so from the ores 
or because it has been previously melted. Those named 
above have found augmentation with the pure lead; 
the sieur Bnm, a diminution with the other. 



"Conclusion. 

"Behold now this truth, whose brilliance strikes the 
eye, which I have drawn from the deepest dungeons 
of obscurity. This it is to which the path has been 
hitherto inaccessible. This it is which has distressed 
with toil so many learned men, who, wishing to know it, 
have striven to clear the difficulties which held it encircled. 
Cardan, Scaliger, Fachsius, Caesalpinus, Libavius, have 



THE THEORIES OF CHEMISTRY. 13 

curiously sought it but never perceived it. Others may- 
be on its quest, but vainly if they fail to follow the road 
which I first of all have made clear and royal; all others 
being but thorny footpaths and inextricable byways 
which lead never to the goal. The labor has been mine; 
may the profit be to the reader, and to God alone the 
glory." 



The great battle, between modem chemistry and the 
believers in the doctrine of phlogiston, began in 1774. 
It reached its zenith diuing the years 1783-89. It did 
not close until 1794. The leader of the new school was 
Lavoisier. 

Priestley discovered oxygen on the first day of August, 
1774, by heating the red oxide of mercury. This new 
gas (oxygen), superior to ordinary air in supporting 
combustion, was named "dephlogisticated air" by its 
discoverer. 

Priestley thought that the non-combustible part of oiu: 
atmosphere was satiurated with phlogiston, on the assump- 
tion that a gas was so much the better adapted for sup- 
porting combustion when it contained within itself a 
small quantity of phlogiston. Common air by drawing 
phlogiston from burning bodies became, as Lavoisier 
thought, phlogisticated air, and on that account exhibited 
no attraction for phlogiston; or in other words, no longer 
had the power of supporting combustion. The phlo- 
giston, evolved in the burning of combustible bodies and 
in the calcination of metals, was supposed to unite with 
the atmosphere, or better with the dephlogisticated air 
contained therein, and the product thus produced was 



14 THE THEORIES OP CHEMISTRY. 

the cause of the increased weight of the calces. Lemery, 
in 1700, advanced the idea that the air given off diiring 
the solution of metals in acids was their combustible 
constituent. Priestley, guided by the thought sug- 
gested by Cavendish — ^that metals in order to unite with 
acids must part with their phlogiston — boldly announced 
that inflammable air [hydrogen] was either identical 
with phlogiston or that it was very rich in that principle. 

In 1781, Cavendish noticed that hydrogen and dephlo- 
gisticated air, when exploded in a closed vessel, in pro- 
portions sufficient to almost entirely phlogisticate the 
burnt air, produced piure water; further that water was 
formed when a mixture of air and inflammable air was 
exploded; and, in addition, that there was a reduction 
of one-fifth of the voltmie of the former noticeable. Ac- 
cording to Cavendish, water consisted of phlogiston and 
dephlogisticated air; inflammable air, of hydrogen 
[or phlogiston] and water. The action of the dephlo- 
gisticated air upon inflammable air, when exploded with 
it, was to unite with its phlogiston and yield water, and 
consequently to set free the water in the inflammable air. 

In 1788, Priestley offered the suggestion that water 
entered into the composition of fixed and other airs, 
in order to account for the formation of water when 
inflammable air and dephlogisticated air united. Inflam- 
mable air, he thought, might be the principle of alkalinity 
and dephlogisticated air the principle of acidity. 

Scheele believed in the existence of phlogiston. While 
experimenting, to ascertain the nature of heat and fire, 
he discovered that measured quantities of common air, 
when kept in contact with certain substances, became 
incapable of supporting combustion. As the specific 
gravity of the air had not augmented the decrease in 



THE THEORIES OF CHEMISTRY. 15 

bulk, Scheele concluded, could not be due, as at first 
conjectured, to the absorption of phlogiston, so that the 
atmosphere must consist of two distinct bodies, one of 
which, the residual air, he assumed to be incapable of 
combining with phlogiston, and the other, having a 
strong attraction for that substance, imited with it, pro- 
ducing heat, which had penetrated through the walls of 
the vessel containing it; hence the diminution of the 
volume of air. 

*'Heat," Scheele said, **was decomposed by means 
of bodies which had a strong attraction for its phlogiston." 
Calces of gold, silver, etc., were reduced by the phlogiston 
in the heat and consequently the other constituent of 
heat, empyreal or fire air [oxygen], was liberated. Sub- 
sequently, when it became impossible to ignore the con- 
sideration of the increase in weight noticed in many 
substances after their combustion or calcination, Scheele 
so far modified his views as to pronounce fire air to be 
a compoimd containing, with very Httle phlogiston, 
a saline principle and water, which last gave to fire air 
the greater part of its weight. When fire air formed 
heat by combining with phlogiston it gave up its v/ater 
to the materials it dephlogisticated, and thus it was 
that they were rendered heavier by ignition. It was 
the water that attached itself to the calces and made 
them heavier. 

The foregoing statements briefly contain the views 
entertained by Cavendish, Priestley and Scheele, three 
of the most able chemists, and not only by them but by 
many others laboring in the field of chemistry at this 
time. 

Lavoisier's experiments and deductions now claim the 
attention. In 1772, he found that sulphur and phos- 



16 THE THEORIES OF CHEMISTRY. 

phorus increased in weight on burning, so he conceived 
a true idea of the nature of combustion before the real 
character of air was solved. In 1774, Lavoisier melted 
tin in a large, closed, air-tight flask, which he had pre- 
viously weighed. When the tin became coated with a 
thin layer of oxide the apparatus was allowed to cool and 
was then reweighed. Its weight wa.s the same as at first, 
but on opening the flask air rushed in and the weight 
of the flask and its contents was then greater. The tin, 
by calcination, had abstracted something from the air, 
and had not, as required by the theory of phlogiston, 
added anything to it. Further, Lavoisier carefully noted 
the relative increase in weight of the tin and decrease 
in weight of the enclosed air; so that in 1775 he published 
a paper entitled **The Principle which Unites with Metals 
during Calcination." In this paper he commented on 
the heating of the calces of metals with charcoal, and 
said that the original metal plus fixed air, or carbon 
dioxide, must be the product of a union of an elastic 
substance with charcoal. Priestley had shown him 
this elastic substance in 1774, when he heated mercuric 
oxide in Lavoisier's presence and described the properties 
of oxygen, the elastic fluid which was evolved on heating 
a calx of mercury. 

Neither the science of chemistry nor yet a change in 
its objects can be said to have originated with Lavoisier. 
The methods he introduced to attain these objects, the 
ideas he put forth concerning the constitution of bodies, 
and the explanations he gave of various phenomena 
were new and gave a new aspect to the science. 

Indeed, the mine of chemistry had yielded rich returns 
before Lavoisier came to labor in it. He merely availed 
himself of the old workings, and, extending them, opened 



THE THEORIES OF CHEMISTRY. 17 

the main lode. Said Liebig, ''Lavoisier discovered 
no new body, no new property, no natiiral phenomena 
previously unknown; but all the facts established by 
him were the necessary consequence of the labors of those 
who preceded him. His merit, his immortal glory, con- 
sisted in this, that he infused into the body of the science 
a new spirit; but all the members of that body were 
already in existence and rightly joined together." 

In his first paper Lavoisier never once indicated that 
he doubted the existence of phlogiston, and as late as 
1775 he spoke of it, but in the following year he did 
express his conviction that for the elucidation of cer- 
tain phenomena one must ascribe to phlogiston other 
qualities than those assigned to it by Stahl. 

A noticeable feature of Lavoisier's work was that he 
weighed. Boyle weighed, and Rey, and many others 
had used the balance. Cavendish had measured earth, 
air, fire and water. But, notwithstanding all this, 
weighing was not so universally resorted to until the 
time of Lavoisier. He weighed everything, and from 1789 
the balance became an instnmient which was in constant 
use by the chemist to test the steps in his experimenta- 
tion, so that it is proper to say that the quantitative era 
of chemistry began with the work of Lavoisier. 

In 1777, Lavoisier combated the assertion of Priestley 
(and this is the first time that he ever mentioned the 
name of the Manchester philosopher) that air is rendered 
irrespirable by becoming loaded with phlogiston, and 
he also demonstrated that ordinary air in which candles 
were burnt furnished about two-fifths of its volume of 
fixed air, and that pure or dephlogisticated air under 
the same conditions became almost completely trans- 
formed into that gas. 



18 THE THEORIES OF CHEMISTRY. 

It was in 1778 that he broached the theory that dephlo- 
gisticated air [oxygen] was the universal acidifying or 
oxidizing principle, which by combination with charcoal, 
with sulphur, with nitrogen, with phosphorus, formed 
carbonic, vitrioHc, nitric and phosphoric acids, and with 
metals yielded calces. All the phenomena of combustion 
were explicable by means of oxygen and without the 
supposition of the existence of phlogiston, of which there 
was absolutely no evidence. The anti-phlogistic theory 
was being gradually evolved, and in 1783, Lavoisier 
declared absolutely against the doctrine of Stahl. It 
was then that he determined to discover the nature of 
the product of the combustion of hydrogen. He had 
heard that Cavendish obtained water when he btimed 
hydrogen. He repeated Cavendish's experiments, and 
likewise foimd that water contained hydrogen. He further 
found that on passing steam through a red-hot porcelain 
tube filled with iron wires that he obtained hydrogen. 
He also discovered that one volimie of oxygen combined 
with 1.9 volume of hydrogen to form water. 

Lavoisier had long been unable to hold the doctrine 
that the hydrogen evolved by metals in acids, was 
their phlogiston. This he believed could not be true, 
since the calces were heavier than the metals, and that 
hydrogen, though light, certainly had weight. He 
explained the production of hydrogen liberated dtuing 
the solution of metals in acids by assimiing that water 
was decomposed, its oxygen imiting with the metals 
while the hydrogen escaped. In the case of nitric acid, 
however, he said the oxygen was probably supplied by the 
acid. This last opinion was the cHnching fact in Lavoi- 
sier's theory — the key to the overthrow of the doctrine 
of Stahl. 



THE THEORIES OF CHEMISTRY. 19 

In 1789, Lavoisier's magnificent treatise on elementary 
chemistry appeared. The new chemical discoveries were 
described with great clearness and ability. The anti- 
phlogistic theory was set forth — demonstrating that 
during combustion there was a combination with oxygen. 
Oxygen was the universal acidifying or oxygenizing 
principle (he made the mistake, however, of laying too 
much stress upon oxygen as the acidifying principle, 
as shall be learned later). In addition, he deduced 
the great Law of the IndestructibiHty of Matter. He, 
too, disproved the idea of the elemental nature of heat, 
laid the foimdation of modem chemistry and defined 
elements exactly. 

One often wonders at this day what effect these state- 
ments of Lavoisier had upon the men of that period. 
Priestley wrote: ''The doctrine of the decomposition of 
water being set aside, that of phlogiston, which in con- 
sequence of the late experiments on water has been almost 
universally abandoned, will much better stand its ground.'* 
From a recent reading of an old pamphlet published in 
1798, in this country, entitled '*A Sketch of the Revolu- 
tions in Chemistry," it may be added that the chemists 
of the United States, although favorably inclined to the 
French School, to the anti-phlogistic notions, yet main- 
tained that until Priestley, was converted to the new 
notions they could not feel safe that the modem ideas 
(as they were then) were absolutely true, because of all 
those contending on this subject, Priestley stood pre- 
eminent as an experimenter. 

Cavendish never abandoned his faith in phlogiston. 
Priestley, who emigrated to the United States and died 
there in 1804, corrected proofs on his death bed of a paper 
entitled "Phlogiston Established." He was the last 
of the believers and defenders of that faith. 



20 THE THEORIES OF CHEMISTRY. 

Berthollet, while accepting the explanation of calcina- 
tion and combustion, did not deem the acidifying prin- 
ciple essential. 

In Germany, national prejudice naturally declared 
in favor of Stahl, but in 1792, after Klaproth performed 
Lavoisier's experiments before the Berlin Academy, 
he and other German chemists were convinced of the 
truth of Lavoisier's ideas, hence it may be said that the 
overthrow of the phlogiston theory in Germany occurred 
in 1792. 

Conversion to the new ideas was not instantaneous 
by any means. It took time. Yet the fate of the Stahlian 
philosophy was sealed. Many tried to support it, but 
eventually Lavoisier's explanations came to be regarded 
as satisfactory in every respect and his anti-phlogistic 
theory was universally accepted as a new system of 
chemistry. 

"It was the glory of Lavoisier," wrote Davy in 1814, 
*'to lay the foundation of a sound logic in chemistry 
by showing that the existence of this principle (phlogiston) 
or other principles should not be assimied where they 
could not be detected and proved to exist." 



From 1790 forward the chemists of the world gradually 
abandoned the idea of phlogiston. As a result of this, 
Lavoisier and his immediate associates in France turned 
their attention to the establishment of the new nomen- 
clature, part of which is used to-day. In conclusion, 
it may be said that this work of Lavoisier completes 
what might be called "the second landmark in the 
history of the development of chemical theory." But 
do not forget what was written by Jean Rey, and bear 



THE THEORIES OF CHEMISTRY. 21 

in mind that it required more than one hundred and fifty 
years to establish a correct explanation of the processes 
of oxidation and of respiration. Things of importance 
are often very long in reaching their fruition. 



In point of time, the next most important advance 
made in theoretical chemistry (the next landmark) was 
the presentation of the chemical atomic theory by John 
Dalton. From the very earliest times philosophers 
believed in the atomic character of matter; but until 
Dalton took this subject into consideration there was 
no real, earnest attempt made to apply it to chemistry. 
Dalton never maintained that he was the author of the 
atomic theory. He was the first to explain the effects 
of chemical combination by a theory which has stood 
the test of time. It is often heard from physicists and 
from those who deal in speculative philosophy that the 
day of the atomic theory has passed. This is not true. 
The day of the chemical atomic theory is still with 
chemists. 

"The observations of the earlier philosophers," Dalton 
said, "has led to the conclusion, which seems imiversally 
adopted, that aU bodies of sensible magnitude, whether 
solid or liquid, are constituted of a vast number of 
extremely small particles." Dalton never grew weary of 
explaining that homogeneous, elastic fluids are con- 
stituted of small particles. In his book on meteorology 
he insisted on the separate existence of aqueous vapor 
from other constituents of the atmosphere. This was 
the first germ of his chemical atomic theory, since he 
viewed the gases as consisting of independent particles, 



22 THE THEORIES OF CHEMISTRY. 

In tracing the development of Dalton's chemical atomic 
theory a few abstracts from his early paper will be given. 
Newton, as evidenced by the first of the quotations, 
furnished inspiration to Dalton. 



'Newton had demonstrated clearly in the 23rd prop- 
osition of Book II of the 'Principia' that an elastic fluid 
is constituted of small particles or atoms of matter which 
repel each other by a force increasing in proportion as 
their distance diminishes. 

"That the particles of elastic fluids are of different 
sizes and weights under like circumstances of temperature 
and pressure having been once established, it became 
an object to determine the sizes and weights together 
with the relative number of atoms in a given volume. 
This led the way to the combinations of gases and to the 
number of atoms entering into such combinations. . . . 
Other bodies besides elastic fluids, namely, liquids and 
solids, were subject to investigation in consequence of 
their combining with elastic fluids. Thus a train of 
investigation was laid for determining the number and 
weight of all chemical elementary principles which enter 
into any sort of combination one with another. 



"In my last lecture," writes Dalton, "we endeavored 
to show that matter, though divisible in an extreme 
degree, is nevertheless not infinitely divisible — that there 
must be some point beyond which we cannot go in the 
division of matter. The existence of these ultimate 



THE THEORIES OF CHEMISTRY. 23 

particles of matter can scarcely be doubted, though they 
are probably much too small ever to be exhibited by 
microscopic improvements. 

'* I have chosen the word * atom ' to signify these ultimate 
particles in preference to 'particle,' 'molecule' or any 
other diminutive term, because I conceive it is much 
more expressive ; it includes in itself the notion of indivis- 
ibility, which the other terms do not. It may perhaps 
be said that I extend the application of it too far when 
I speak of compoim.d atoms; for instance, I call an 
ultimate particle of carbonic acid a compound atom. 
Now, though this atom may be divided yet it ceases 
to become carbonic acid, being resolved by such division 
into charcoal and oxygen. Hence I conceive there is 
no inconsistency in speaking of compoimd atoms, and 
that my meaning cannot be misimderstood. 

"It has been imagined by some philosophers that all 
matter, however unlike, is probably the same thing, and 
that the great variety of its appearance arises from cer- 
tain powers commimicated to it and from the variety of 
combinations and arrangements of which it is suscep- 
tible. From the notes I borrowed from Newton this does 
not appear to have been his idea. Neither is it mine. 
I should apprehend that there are a considerable ntmiber 
of what may properly be called elem^entary principles 
which never can be metamorphosed into one another 
by any power we can control. We ought, however, 
to avail ourselves of every means to reduce the number 
of bodies or principles of this appearance as much as pos- 
sible; and after all we may not know what elements 
are absolutely indecomposable and what are refractory, 
because we do not apply the proper means for their 
reduction. We have already observed that all atoms 



24 THE THEORIES OF CHEMISTRY. 

of the same kind, whether simple or compound, must 
necessarily be conceived to be alike in shape, weight, 
and every other particular. 



*'In the investigations on the number and weights of 
the elementary principles constituting water, ammonia, 
and the various compounds of azote and oxygen, con- 
clusions were derived principally from the facts and 
experience of others, without any additional facts of my 
own discovery that merit particular notice. 

"The composition and decomposition of water had 
been ascertained by British and foreign chemists; that 
of ammonia, by Berthollet and several others. The 
compounds of azote and oxygen had been successively 
developed by Cavendish, Priestley, Davy and others. 
I may, however, observe that the nitrous compotmds 
have occupied a great portion of my time and attention 
at different seasons. 

''The simple and easy method of combining the least 
portion of oxygen with the greatest of nitrous gas, . . . 
was the result of my own investigation, and affords a 
convincing proof of the real nature of what is called 
nitrous acid, which is constituted of one atom of oxygen 
imited to two of nitrous gas. 

**From the preceding remarks it will be perceived 
that I advanced thus far in my theoretical progress with- 
out meeting with much obstruction. The way had been 
paved by others. But when I directed my views to the 
compounds of charcoal and oxygen, and charcoal and 
hydrogen, I found that all the commonly received doc- 
trines were adverse to my proceeding and irreconcilable 
with my views. 



THE THEORIES OF CHEMISTRY. 25 

"Mr. Termant's experiments in the Phils. Transact., 
1797, has shown the identity of diamond and charcoal 
in a chemical point of view; but the succeeding experi- 
ments of Guyton Morveau on the combustion of diamond 
supplanted the former in the judgment of a great part 
of our chemists; diamond was concluded to be a simple 
body and charcoal the oxide of diamond. Mr. Cruick- 
shanks soon after discovered the gas called carbonic 
oxide. The doctrine of the compounds of charcoal, 
or rather diamond and oxygen, then stood thus: 

Parts. 

Diamond 18 

Oxygen 10 

28 Charcoal 
Oxygen 41 

69 Carbonic Oxide 
Oxygen 31 

100 Carbonic Acid 

*'A very little reflection convinced me that the doctrine 
of charcoal being an oxide of diamond was highly improb- 
able; and experience confirmed the truth of Lavoisier's 
conclusion that 28 parts charcoal plus 72 of oxygen con- 
stitute carbonic acid, also that carbonic oxide contained 
just half the oxygen that carbonic acid does, which indeed 
had been determined by Clement and Desormes, two 
French chemists, who had not, however, taken notice 
of this remarkable result." 



26 THE THEORIES OF CHEMISTRY. 

In the last lecture of this course Dalton explained his 
singular and, as it turned out, altogether erroneous views 
respecting- the chemical nature of chlorine, hydrochloric 
and hydrofluoric acids. 



"Lecture 20, February 3, 1810. — Chemical 
Elements. 

*'When we consider the very important part which the 
two elementary atoms of hydrogen and oxygen seem to 
perform in the arrangement of chemical compounds we 
are inclined to wonder that no more than one compound 
of these two elements themselves should be found. 

** Water, that most beneficial essential of all liquids, 
is formed of oxygen and hydrogen. Besides this one there 
is not a compound of these two elements generally known 
and recognized as such. It is singular if we have not 
somewhere a principle consisting of two atoms of oxygen 
and one of hydrogen or two of hydrogen and one of 
oxygen. The former of these ought to be an acid, con- 
formably to what we observe in other similar cases; the 
latter ought to be a combustible gas. All the other 
common elements: azote, charcoal, sulphur and phos- 
phorus, combine each one with two atoms of oxygen to 
form acids. Why should not hydrogen do the same? 
This question has been frequently put, but no satisfactory 
answer has been given. Upon comparing the results of 
experience and applying the theoretic views which I have 
been endeavoring to develop, it appears to me very 
probable at least that the acids denominated fluoric and 
muriatic, with derivatives, are constituted of the elements 
of hydrogen and oxygen, and are in reality the very 



THE THEORIES OF CHEMISTRY. 27 

compounds of which we have just been hinting. I would 
not, however, be understood to mean that these views are 
the necessary result of the Atomic Theory, and that its 
truth or falsehood depends upon the determination of the 
question. From the want and imperfection of facts 
relating to these subjects nothing, perhaps, decisive can 
be yet advanced. I intend to point out such reasons and 
such facts as have induced me to adopt this opinion, and 
must leave it to others to judge how far they support the 
probabilities above mentioned." 



Then Dalton gives short memoranda of the anal3rtic 
and other grounds upon which he based his views. He 
concludes his course in the following words: 

**I cannot conclude this course of lectures without 
expressing my high satisfaction with the general attention 
that has been given to the subjects under discussion, and 
with the indulgence which has been given me when adverse 
circumstances oconred. I shall always associate these 
grateful impressions with the recollection of the event. 
To those who feel highly interested themselves in the 
promotion and extension of science it is a peculiar satis- 
faction to meet with others of the same description. I 
shall now return to comparative retirement in order to 
prosecute the train of enquiry and investigation which I 
have briefly developed in the late lectures. The results, 
I am confident, will be found of importance, and will con- 
tribute to establish that beautiful and simple theory of 
chemical s>Tithesis and analysis which I have adopted 
from a conviction of its application to the general phenom- 
ena of chemistry, and which will in due time, I am per- 



28 THE THEORIES OF CHEMISTRY. 

suaded, be made the basis of all chemical reasoning 
respecting the absolute quantities and the proportions of 
all elementary principles, whether simple or compound. 
*'R. I., Feb. 3rd, 1810." 

From these extracts it is clear that Dalton was a 
thorough-going atomist. He was deeply steeped in New- 
tonianism. For him the atoms, *'hard, impenetrable, 
movable," had as actual an existence as if he had seen 
and handled them. He first saw in his mind's eye the 
atoms of oxygen, of azote and of watery vapor existing 
in the air. He drew them on paper, and tried to combine 
them so as to account for the homogeneous character 
of the atmosphere. In this endeavor he failed, and at 
once had recourse to another hypothesis, namely, that 
these atoms are not chemically combined, but that the 
particles of one gas act as vacua to those of another, thus 
accounting for the laws of diffusion which he had dis- 
covered some time previously. Then he concludes that 
the sizes of the atoms of different gases must be different, 
and **it became an object to determine their relative 
sizes and weights." And thus the foundation of the 
atomic theory was laid. But still the superstructure 
remained to be built up. Here agaia we can trace his 
further progress. Early in 1803, Dalton stated that the 
elements combine together in definite proportions to form 
compounds, for example: one atom of nitrogen with one 
of oxygen, and in no intermediate proportions. Dalton 
doubtless asked himself, ''Why are these things so?" 
And the answer was not far to seek. If these gases are 
made up of small indivisible particles, and if chemical 
combination consists in the approximation of these par- 
ticles — ^the clashing of the atoms, as it is now termed — 



THE THEORIES OF CHEMISTRY. 29 

then it is clear that when two elementary gases, as oxygen 
and nitrogen, combine, the simplest form of the compound 
is that produced by one atom of the first coming into 
juxtaposition with one atom of the second gas. If any 
further combination is possible^ — and that possibility is 
not foretold by Dalton's theory, but is a matter to be 
decided by experiment- — such a new combination can 
only be brought about by the juxtaposition of a second 
atom of one of the combining elements. 

In Dalton's Laboratory Notebook for 1802-04, in the 
possession of the Manchester Society, there is the follow- 
ing entry on Septem^ber 6th, 1803 : 

"Observations on the ultimate particles of bodies and 
their combinations . ' ' 

And under this is written : 

"Characters of Elements. 
Hydrogen Azote 

O Oxygen # Carbone, pure charcoal 

© Sulphur" 

And a few pages farther on Dalton gives examples of 
his method of expressing the composition of chemical 
compounds : 



0O® 


Nitrous Oxide 


O O Water 


(DO 


Nitrous Gas 


O Ammonia 


O0O 


Nitric Acid 


O • Gaseous Oxide 


O0O| 

0O 


Nitrous Acid 


of Carbon 
O • O Carbonic Acid 


©o 


Sulphurous Acid 


O © O Sulphuric Acid" 



30 THE THEORIES OF CHEMISTRY. 

On another page of this notebook, with the date 
October 12, 1803, there is the following: 

"New theory of the constitution of the ultimate atoms 
of bodies. 

Characters or thu8 

@ Hydrogen O 

® Azote ® 

O Oxygen O 

• Carbone or charcoal 9 

© Sulphur © 

Phosphorus O" 

And these are interesting as being the very first 
attempts to indicate by S3niibol the atomic construction 
of chemical substances. 

In the chapter on ** Chemical Synthesis" in Dalton's 
*'New System" he explains the symbolic language which 
he uses : 

**From the novelty as well as the importance of the 
ideas suggested . . . it is deemed expedient to give 
plates exhibiting the mode of combination in some of the 
more simple cases. . . . The elements or atoms of 
such bodies as are conceived at present to be simple are 
denoted by a small circle with some distinctive mark, 
and the combinations consist in the juxtaposition of two 
or more of these. When three or more particles of elastic 
fluids are combined together in one it is to be supposed 
that the particles of the same kind repel each other and 
therefore take their stations accordingly." 

But the most important part of Dalton's theory remains 
to be considered; and it is this part which especially 
distinguishes it from all previous conceptions of the 



THE THEORIES OF CHEMISTRY. 31 

constitution of matter. It was, "an enquiry into the 
relative weights of the iiltimate particles of bodies — a 
subject," he adds, "as far as I know, entirely new. I 
have lately been prosecuting this enquiry with remarkable 
success." The weights of the atoms are different. The 
atom of oxygen is — according to Dalton, in the year 
1803 — rather more than five times heavier than the atom 
of hydrogen; that of nitrogen (azote) four times heavier, 
and so on; but every atom of the same element has the 
same unalterable weight. 

The first pubHshed table of atomic weights was that 
appended to Dalton 's "Memoir on the Absorption of 
Gases in Water," read before the Society on October 21, 
1803. There is in Dalton 's notebook an earlier table 
dated September 6th of that year, giving numbers differ- 
ing somewhat from those of the printed table. 

"Table of Atomic Weights — September 6, 1803. 

Ultimate Atom of Hydrogen 1. 

Oxygen 5.66 

Azote 4. 

" " Carbon (charcoal) 4.5 

" " Water 6.66 

" " Ammonia 5. 

" " Nitrous Gas 9.66 

" " Nitrous Oxide 13.66 

Nitric Acid 15.32 

Sulphur 17. 

" " Sulphurous Acid 22.66 

Sulphuric Acid 28.32 

Carbonic Acid 15.8 

" " Oxide of Carbon 10.2 " 

It will be observed that no mention is here made of 
the two compoimds of carbon and hydrogen, the names 



32 THE THEORIES OF CHEMISTRY. 

and compositions of which are given in the table printed 
in the above-named memoir. Hence it is clear that 
Dalton had arrived at conclusions respecting the atomic 
weights of certain elements and of some of their com- 
pounds before the summer of 1804, when, according to 
his own statement, he carried out a further experimental 
inquiry into the composition of two compounds of carbon 
and hydrogen, namely, olefiant gas and light carburetted 
hydrogen, which led to results fully confirming the truth 
of his theory. These experiments showed that, as in the 
case of nitrous oxide and nitrous gas and in that of 
carbonic acid and the oxide of carbon, so, too, in the case 
of *' carburetted hydrogen from stagnant water" and 
olefiant gas a simple multiple proportion holds good 
between the weights of the constituent elements; that is, 
the first of these gases contains just twice as much 
hydrogen to the same weight of carbon as the second 
(olefiant gas) does; and hence Dalton represented them 
by the symbols © # © and # © respectively. This 
conclusion was, however, not arrived at till 1804, and 
yet the nimibers for the two hydrocarbons (6.3 and 5.3) 
appear in the table attached to a memoir of October, 
1803. The explanation of this is that the volume of 
memoirs was not published till November, 1805, and that 
in the meantime Dalton made additions and alterations 
to the table. "It was in the summer of 1804," says 
Dalton, "that I collected at various times and in various 
places the inflammable gas (marsh gas) obtained from 
ponds." Those who wish to see how Dalton did this 
should visit the Manchester Town Hall and inspect the 
wonderful fresco by Madox Brown, where the process is 
clearly indicated. 
The circular s3^mbols used by Dalton are not employed 



THE THEORIES OF CHEMISTRY. 33 

by chemists to-day. The elements are represented by 
the first letter or the first letter and one other letter of 
the name of the element (sometimes the modem, some- 
times the Latin name) ; a small number placed below the 
symbol indicates the fact that more than one atom is 
present. Thus Dalton's two hydrides of carbon, at his 
time, would have been written CH2 and CH, now they are 
written CH4 and C2H4. This modem system was intro- 
duced by the Swedish chemist, Berzelius. Each symbol 
carries with it, a weight value. Thus H does not signify 
an indefinite weight of hydrogen but always one part by 
weight, and so for all the other elements, each having its 
own atomic weight understood. And so, too, for their 
compounds — the formiila of a chemical compoimd not 
only indicates of what elements it is composed but also 
shows how much of each element is contained in it. The 
language of chemistry is, therefore, not merely qualitative 
but also quantitative. 

It is a singular trait of Dalton's mind that he could not 
bring himself to adopt, or, if one may say so, even to 
understand this system of chemical nomenclature, appar- 
ently so simple and effectual. On the contrary, he con- 
sidered it to be unscientific. 

Dalton, to the end, adhered to the syrabolic language 
which he introduced in 1802. That this was so is seen 
from a facsimile table of "Atomic Symbols" drawn up 
by him for a lecture delivered in the Manchester Mechan- 
ics' Institute on October 19, 1835. Writing to Graham 
so late as 1837, he says: 

"Berzelius's symbols are horrifying; a young student 
in chemistry might as soon learn Hebrew as make himself 
acquainted with them. They appear like a chaos of atoms 
. . . and to equally perplex the adepts of science, to 



34 THE THEORIES OF CHEMISTRY. 

discourage the learner, as well as to cloud the beauty and 
simplicity of the Atomic Theory." 

Dalton's atomic hypothesis was brought prominently 
before the world of science, not by the author himself, 
but by an influential professor of chemistry, Dr. Thomas 
Thomson, of Glasgow. In August, 1804, Thomson spent 
some days with Dalton in Manchester, and as a result 
of their conversation Thomson became an ardent disciple 
of the new doctrine, which he taught at once publicly 
in his lectures and then introduced into his text-book 
published in 1807. In his ''History of Chemistry" 
Thomson says: "Mr. Dalton informed me (1804) that the 
atomic theory first occurred to him during his investi- 
gation of defiant gas and carbiuretted hydrogen gas, 
at that time imperfectly imderstood and the constitution 
of which was first developed by Mr. Dalton himself. 
. . . He found that if we reckon the carbon in each 
the same, then carburetted contains exactly twice as much 
hydrogen as olefiant gas. This determined him to state 
the ratios of these constituents in numbers, and to con- 
sider the olefiant gas a compound of one atom of carbon 
and one atom of hydrogen, and carburetted hydrogen 
of one atom of carbon and two atoms of hydrogen. The 
idea thus conceived was applied to carbonic oxide, water, 
ammonia and other substances, and numbers represent- 
ing the atomic weights of oxygen, azote, etc., were deduced 
from the best anal3rtical results which chemistry then 
possessed." 

From what has been stated as to the genesis of Dalton's 
ideas, it is clear that this statement of Thomson is not 
quite exact. Dalton had formiilated his theory and 
actually given numbers representing the relative weights 
of the atoms of certain of the elements and their com- 



THE THEORIES OF CHEMISTRY. 35 

pounds in the early autumn of 1803, before he had inves- 
tigated the composition of the two hydrides of carbon, 
which according to his own statement, he did not do until 
the summer of 1804. 

Observe that throughout his work Dalton uses the 
term "relative weight." More than one hundred years 
ago the author of the atomic theory says nothing concern- 
ing the absolute weight of the atoms; and at present, 
the weights of atoms are ''relative." No mortal eye 
has ever seen or ever will see an atom. Some notion 
of their probable size, however, can be gathered from the 
following : if a drop of water were to be magnified to the 
size of the earth the molecules of water would be larger 
than small shots but smaller than cricket balls. 

How, then, did Dalton ascertain that the atoms of the 
different elements, being inconceivably minute, are not 
of the same weight, but that to each a definite number 
may be attached? In the first place, Dalton decided 
to compare the different atoms with hydrogen, taken 
as a imit, since hydrogen is the lightest known substance. 
It is then clear that this relation, existing between the 
weights of the atoms, must be obtained by calculation 
from the results of chemical analysis or synthesis of cer- 
tain compounds of the elements. But on what principle 
was this calciilation based ? The principle or assimiption 
adopted was that when only one compound of two ele- 
ments is known to exist that compound consists of one 
atom of each element. That is the simplest conceivable 
arrangement, and the relative quantities with which these 
elements combine to form the compound are the least 
combining weights of the two elements. 

It was found that 100 parts by weight of water con- 
tained 14 J parts by weight of hydrogen and 85 1 parts 



36 THE THEORIES OF CHEMISTRY. 

by weight of oxygen, or the elements were present in the 
proportion of 1:6; since Dalton's standard for atomic 
weight determinations was the hydrogen atom = l, these 
numbers (1:6) were taken by him as representing the 
relative weights of the atoms of hydrogen and oxygen — 
water consisting of one of each. 

In a similar way, the relative weights of the atoms of 
nitrogen and hydrogen were obtained. Ammonia gas 
is a compound of these two elements, and was supposed 
to consist of one atom of each of these. Analysis proved 
that it contains hydrogen and nitrogen in the proportion 
of 1 : 4.2 parts by weight. Hence this latter number is the 
atomic weight of nitrogen. Then, assimiing this number 
for the atom of nitrogen, and knowing the composition of 
the two oxides of nitrogen, it is easy to calculate the atomic 
weight of oxygen in these two gases, and thus to show 
that oxygen enters into every combination with a definite 
fixed weight. This gives 4.9 as the relative weight of 
the atom of oxygen combined with nitrogen, against 
6.0 when combined with hydrogen. The mean of these 
two numbers is nearly 5.5, a number adopted by Dalton 
in his first published table of atomic weights. In a 
similar manner the weight of the atom of carbon was 
found to be 4.5. The niunbers obtained represent, then, 
the relative weights of the atoms and indicate the pro- 
portions by weight with which the elements combine to 
form definite chemical compounds. 

Dalton subsequently made considerable changes in 
his table of atomic weights. This shows that, as time 
went on, he obtained other and more reliable data for 
his calculations. His first results were but rude approxi- 
mations; his subsequent ones, though far from exact, 
were rather more in accord with present values. Indeed, 



THE THEORIES OF CHEMISTRY. 37 

Dalton expressly states this: *'It is not necessary to 
insist on the accuracy of all these compounds both in 
number and weight ; the principle will be entered into 
more particularly hereafter, as far as respects individual 
results." 

Nevertheless, in spite of his rough methods of experimen- 
tation, Dalton's results stand out as the greatest land- 
mark of our science. His great achievement was that 
he was the first to introduce the idea of quantity into 
chemical language. It has been said, and with truth, 
that the atomic theory is almost as old as the hills; 
but no one before Dalton used the theory of atoms to 
explain chemical phenomena. To him is also due the 
glory of placing the science on a firm basis by showing 
that the weights of the atoms of the different elements 
are not identical but different. And, further, that com- 
binations of these elements take place, if more than one 
compoimd be formed of the same elements, in simple 
arithmetical proportion. 

Sir Himiphrey Davy was much slower than Thomson 
to adopt Dalton's results. He was full of his discovery 
of the decomposition of the alkalies, and he writes to 
Dalton, in 1809, that while he is glad to hear his new 
views of the atomic system, yet he "doubts whether we 
have yet obtained any elements;" and in a letter to 
Dalton, dated May 25, 1810, Davy writes: 

"I shall be sorry if you introduce into your rising 
system an hypothesis which cannot last concerning the 
alkaline metals." In 1811, he fiurther objects: *'I shall 
enter no further at present into an examination of the 
opinions, results and conclusions of my learned friend. 
I am, however, obliged to dissent from most of them and 



38 THE THEORIES OF CHEMISTRY. 

to protest against the interpretations that he has been 
pleased to make of my experiments." 

That Dalton was sanguine as to Davy's conversion 
appears from a letter addressed to Mr. Johns, in December, 
1809. ''Davy is coming very fast to my views on chem- 
ical subjects." And in 1826, he seems to have fully 
accepted his views, for in that year, Davy, then President 
of the Royal Society, spoke in the highest terms of Dalton's 
services to science when he presented the Royal Medal 
to him. 

Soon after the volume of Dalton's *'New System" 
was in the hands of chemists a discovery was made in 
France which, to the minds of almost all, came as a further 
striking proof of the truth of the Daltonian hypothesis. 
Strange to say, however, Dalton was about the only man 
to whose mind the experiments and conclusions of Gay- 
Lussac did not bring further confirmation of the atomic 
theory. Briefly stated Gay-Lussac's deductions were: 
that under similar circumstances of temperature and 
pressure, gases combine together in simple proportions 
by volume. Two volumes of hydrogen gas and one 
volume of oxygen gas unite together to form two volumes 
of water-gas. That Gay-Lussac's results were at once 
brought before Dalton's notice appears from a letter in 
the archives of the Manchester Society, from Dr. Thomas 
Thomson to Dalton, dated November 13, 1809: ''The 
most important paper respecting your atomic theory 
is by Gay-Lussac. It is entirely favourable to it, and it 
is easy to see that Gay-Lussac admits it, though respect 
for Berthollet induces him to speak cautiously. His 
paper is on the combination of gases. He finds that 
all unite in equal bulks, or two btilks of one to one of 
another, or three bulks of one to one of another." Thorn- 



THE THEORIES OF CHEMISTRY. 39 

son then gives a list of the combining volimies of different 
elementary gases, as ascertained by Gay-Lussac, and 
adds that the French chemist declares that Dalton's 
experiments on this subject are incorrect. 

Moreover, it appears from the following letter to his 
brother Jonathan, dated *'12mo., 11th, 1809," that 
Dalton had in his possession the very volimie which 
contained Gay-Lussac's celebrated memoir: 

** About two months ago I received a very handsome 
present from Berthollet, sent me in return for mine sent 
him. It is 'Memoirs de la Societe d'Arcueil,' being 
the most recent transactions of the Parisian chemists. 
It contains some very valuable papers. They speak 
very respectfully of my First Part." (Of his New System 
of Chemistry.) 

In a similar strain Berzelius, who at once adopted 
the Daltonian theory, urges him in a letter dated from 
London, on October 13, 1812, to recognize the powerful 
corroboration of his system effected by the researches 
of Gay-Lussac. The words of the great Swedish chemist 
are as follows : 

"Vous avez raison en ce que la theorie des proportions 
multiples est une mystere sans I'hypoth^se atomistique, 
et, autant que j'ai pu m'apercevoir, tous les resultats 
gagnes jusqu'ici contribuent a justiner cette hypothese. 
Je crois cependant qu'il y a des parties dans cette theorie, 
telle que la science vous la doit a present, qui demandent 
a ^tre un peu alterees. Cette partie, p. ex., qui vous 
necessite de declarer les experiences de Gay-Lussac sur 
les volumes des gases qui se combinent pour inexactes. 
J'aurais cru plutot que ces experiences etaient la plus 
belle preuve de la probabilite de la theorie atomisque, 
et je vous avoue d'ailleurs que je ne croirai pas si aisement 



40 THE THEORIES OF CHEMISTRY. 

Gay-Lussac en defaut, surtout dans une mati^re o^ il 
ne s'agit que de m^surer bien ou mal" (sic). 

But Dalton remained obdurate, and as Angus Smith 
remarks, he does not appear to advantage in the contest 
in which he ventured to engage with Davy and Gay- 
Lussac. Dalton, it must be admitted, was not an exact 
experimental chemist. Although it may be urged that 
he was self-taught and began his work when the resources 
of the experimentalist were scanty and imperfect, yet, 
it is evident that there must have been some inherent 
deficiency, either in his mind or in his hands, which dis- 
qualified him for accuracy in experimentation. ** Nature, 
it woiild seem, with wise frugality, averse to concentrate 
all intellectual excellencies in one mind, had destined 
Dalton exclusively for the lofty rank of a law-giver of 
chemical science." 

The question has often been debated as to what extent 
Dalton supported Avogadro's law, namely, that the 
ntimber of particles contained in a given volume is 
constant for all gases whether elementary or compoimd. 
Dalton's own words, contained in some manuscript notes 
for the lectures he delivered in Edinburgh and Glasgow, 
in 1807, leave no room for further question. The third 
lecttire commences as follows: ** Elastic fluids; mode of 
conceiving them. Are the particles alike in shape, weight, 
etc.? Query — are there the same number of particles 
of any elastic fluid in a given volume and under a given 
pressure? No! Azotic and oxygen gases mixed in equal 
measures give half the number of particles of nitrous 
gas, nearly in the same volume." 

On the progress which atomic chemistry has made since 
Dalton's time, and especially in the last few years, it is 
not the province of this volume to dilate. Suffice it to 



THE THEORIES OF CHEMISTRY. 41 

say, that the extraordinary complications in chemical 
combinations, which have been brought to light by modem 
research, all receive their explanation by the application 
of the principles of the Daltonian atomic theory. With- 
out such a theory, modem chemistry would be a chaos; 
with it, order reigns supreme and every apparently 
contradictory and recondite discovery only marks out 
more distinctly the value and importance of Dalton's work. 

In closing this sketch of Dalton's greatest discovery, 
it may be interesting to read the opinions of a famous 
Englishman and of a distinguished Frenchman on the 
life-work of the Manchester philosopher: 

"The extreme simplicity," writes Sir John Herschel, 
** which characterizes the atomic theory, and which in 
itself is an indication, not imeqiiivocal, of its elevated rank 
in the scale of physical truths, had the effect of causing 
it to be annoimced by Mr. Dalton in its most general 
terms, on the contemplation of a few instances, without 
passing through subordinate stages of painful inductive 
ascent by the intermedium of subordinate laws. Instances 
Hke this, where great, and indeed immeasurable, steps 
in otir knowledge of nature are made at once, and almost 
without intellectual effort, are weU calculated to raise 
our hopes of the future progress of science, and by point- 
ing out the simplest and most obvious combinations — as 
those which are actually iovmd to be most agreeable 
to the harmony of creation — to hold out the cheering 
prospect of difficulties diminishing as we advance, instead 
of thickening around us in increasing complexity." 

Wurtz, in his "Histoire des Doctrines Chimiques," 
after indicating the work done by Wenzel and Richter, 
says: 

"Mais I'interpretation theorique faisait encore d^faut. 



42 THE THEORIES OF CHEMISTRY. 

Elle decoule des travaux d'un savant anglais qui a dote 
la science de la conception a la fois la plus profonde et la 
plus feconde parmi toutes celles qui ont surgi depuis La- 
voisier. Au commencement de ce siecle la chimie etait 
professee k Manchester par un homme qui joignait k un 
armour ardent de la science, cette noble fierte du savant 
sait preferer I'independance aiix honneurs, et k une 
vaine popularite la glorei des travaux solides. Ce pro- 
fesseur est Dalton; son nom est tin des plus grands 
de la chimie." 

In the case of almost every great scientific discovery, 
many men's minds have been working in the same direc- 
tion, and it often becomes a question of interest to discuss 
how far the acknowledged discoverer had been assisted 
or even anticipated by those who had gone before him. 
Such a discussion has been raised in this instance. Some 
have even asserted that Dalton was a plagiarist, and that 
the credit of the establishment of a chemical atomic theory 
belonged to others. This is not the place to discuss the 
question at length. Those who desire to make themselves 
fully acquainted with the facts and arguments may turn 
to the pages of either Henry or Angus Smith, where they 
will find the whole matter clearly and satisfactorily dealt 
with. It must suffice here to state that a careful con- 
sideration of all the circimistances has led to the conclu- 
sion, that Dalton arrived independently at his important 
results. 

Others had expressed opinions and gathered facts to 
support these opinions, which approached the complete 
theory, but they certainly did not reach it. Mr. William 
Higgins, an Oxford man, in 1814, claimed to be the right- 
ful owner of the atomic theory, by virtue of the statements 
contained in a volume published in 1789. It appears 



THE THEORIES OF CHEMISTRY. 43 

certain that Dalton was iinacquainted with Higgins' 
work until several years after he published his "New 
System," and thus the charge of plagiarism is fully dis- 
posed of. Indeed, no one who has appreciated Dalton 's 
character could imagine that he would be guilty of such 
a proceeding. He was a man of original ideas, and paid 
little attention to the work of others, often to the extent 
of refusing to admit truths which were apparent to other 
minds. Still it might be that Higgins and others had the 
priority in the conception of the atomic theory and then 
in the history of science Dalton would have to take the 
second place. Such is, however, not the verdict of 
either contemporary or of later chemists all the world 
over. 



Further, it may be asked, was Dalton alone in the 
discovery of the law relating to the definite combination 
of elements by weight, and of the law of mtdtiple propor- 
tion? J 

It may truthfully be affirmed that Dalton developed 
these fimdamental laws of our science, while ignorant of 
the fact that several others before him had had similar 
ideas in regard to chemical combination, and had for- 
mulated conclusions similar to his own but reached by 
entirely different paths. The work of Dalton is largely 
speculative — experimentally it is weak; the other scien- 
tists, who called attention to the definiteness of the com- 
bining power of elements and whose work antedated 
that of Dalton, were experimentaHsts, who did little 
philosophizing, but who recorded experiments which led 
to the same goal as that attained by John Dalton, 



44 THE THEORIES OF CHEMISTRY. 

In 1777, Wenzel published a book entitled "Lehre von 
der Verwandschaft der Korper," in which the author 
describes "Decomposition by Double Transposition." 
The changes occurring were quantitatively determined, 
and this appears to be the first work of its kind. Con- 
sidering the period in which this work was done, the results 
are really admirable. Wenzel confirmed, by his exper- 
iments, the theoretical ideas which had arisen: that 
whenever two neutral salts come together, double trans- 
position takes place, and the salts which are formed 
are invariably neutral. He, also, explained this phenome- 
non. Suppose A and B to be the salts undergoing 
decomposition, and that A contained the base (a) and 
the add (6), while B contained the base (c) and the acid 
(d). If the quantity of the base (a) in the salt A sufficed 
to completely neutralize the add {d) in the other salt, 
B, conversely, the base {c) of the salt B woiild be suffi- 
dent for the complete neutrahzation of the add (b) in 
the salt A. 

Ca(N03)a+K2S04 = 2KN03+CaS04 

The resulting salts were neutral, for the reasons that 
have just been given, namely, that there was sufficient 
nitric acid in this first salt to neutralize the base of the 
second salt when it was liberated, as there was also 
suffident acid in the second salt to neutralize the base of 
the first salt upon its liberation. 

Please note that while, to-day, this is so simple, so self- 
evident, at the time Wenzel began his work it was purely 
speculative. Wenzel confirmed his assertions by many 
additional analyses. He concluded from them that the 
various but definite quantities of different bases, required 



THE THEORIES OF CHEMISTRY. 45 

for the saturation of a definite amount of one and the 
same acid, must be exactly in the same proportion as 
would be required to neutralize definite amoimts of any 
other acid. The principle imderlying the doctrine of 
equivalents is imdoubtedly manifested in Wenzel's conclu- 
sions. His fate was that of so many, namely, that he 
was not recognized as one of the authorities of his day, 
and hence his investigations did not receive the attention 
and recognition they merited. Further, his results were 
compared with the analyses, faulty though they were, 
of the supposed authorities of that time, and as they did 
not harmonize therewith, they were pronoimced erroneous 
and forgotten. 

Some years later (1782) Bergmann observed that, 
when a metal was precipitated from the solutions of its 
salts by another metal, the metal in solution contained 
just enough oxygen to combine with the other metal and 
bring it into the form of an oxide, so that it would dissolve. 
If a strip of zinc is introduced into a copper sulphate solu- 
tion, copper will be precipitated. In the judgment of 
Bergmann the copper was precipitated because it con- 
tained enough oxygen, so that when it came in contact 
with the zinc, its oxygen was sufficient to combine with 
the zinc, and the zinc oxide united with the SO3 to form 
zinc sulphate. Here, again, is the idea of equivalents, and 
the idea of definiteness of combination. Unfortunately, 
Bergmann announced his observations in the language of 
the phlogistians, so that the explanations which he gave 
were wholly inadequate, difficult to understand, and the 
doctrine of definite ratios in combinations remained 
undiscovered. 

But it remained for J. B. Richter, a German chemist, to 
review the work of Wenzel, and by his own experimenta- 



46 THE THEORIES OF CHEMISTRY. 

tion to push forward these investigations ; he brought them 
together in an exceedingly suggestive volume, entitled 
*'Anfangsgrunde der Stochiometrie," published in 1792 
to 1794. It contained the following declarations: First, 
"when two neutral compounds react upon one another, 
a transposition occtirs, and the resulting products will 
always be neutral." That proposition had been demon- 
strated by Wenzel. The second principle annotinced by 
Richter read, "when, on the other hand, one of the two 
compounds reacting upon one another is not neutral then 
one of the products will not be neutral." Third, "it will 
be discovered, that upon determining the ratios of some 
neutral salts, it becomes possible, by simple equations, to 
calculate the composition of other neutral salts." Richter 
gave in tabular form the quantities by weight according 
to which the best known acids and bases \mite with each 
other to form neutral salts. This, he called "the mass 
series or neutralization series;" 1000 parts by weight of 
sulphuric acid constituted the standard, to which all other 
members were referred. The numbers were found by 
comparing the quantity of any one base (A, for example) 
which was sufficient for the saturation of 1000 parts of 
sulphinic acid, and then calculating how much of another 
acid, B, the found quantity of A would neutralize, and 
from the quantity of B, was calculated the quantity of a 
base, G, which would be neutralized by that particular 
quantity of acid, and so on through the series. 

In order to comprehend fully the importance of what 
Richter did, it is well to present a few examples from his 
work. With sulphuric acid as the basis and its value set 
at 1000, Richter proceeded to study calcium sulphate, 
and according to the results of his analysis of that body, 
there was present in it, 55.77 parts sulphuric acid (SO3) 



THE THEORIES OF CHEMISTRY. 47 

and 44.22 parts calcium oxide (CaO). He found the 
ratio number, as he termed it, by means of the pro- 
portion : 

SO3 : CaO :: SO3 : CaO 

55.77 : 44.22 :: 1000 : X 
X = 793 (the value for CaO) 

The value for nitric acid followed from the value that 
had been obtained for the lime, 793, and from the analysis 
of calcitim nitrate, which, according to Richter, contained 
63.92 parts nitric acid and 36.07 parts lime. 

CaO : NaOe :: CaO : NaOg 
36.07 : 63.92 :: 793 : X 
X = 1405 (the value of nitric acid) 

The value for soda was determined from the nitric 
acid value (1405) and from the analysis of sodiimi nitrate, 
made by Richter, according to which it contains 62.05 
parts nitric acid and 37.94 parts soda. The calculated 
soda value was 859. The value for hydrochloric acid 
was obtained from the soda value (859) and it proved 
to be 712. That for potassium was similarly calculated 
from the found value of hydrochloric acid and was dis- 
covered to be 1605. The value of sulphiuic acid finally, 
if it had not already been known, might be calculated 
from this value of potassium (1605) and from the com- 
position of potassiimi sulphate. It proved to be by 
actual experiment 1000. 

A few of Richter's tables are subxTiitted. It may 
appear, at first, that they have no bearing on the laws 
announced by John Dalton, but, upon investigation it 
becomes manifest that there is an intimate connection 
between them and the results of Dalton. 



48 THE THEORIES OF CHEMISTRY. 



Mass or Neutralization Series for Bases and Acids 



HP 


427 


CO2 


577 


WO3 


700 


HCl 


712 



Alumina 525 

Ammonia 609 

Magnesia 614 

Lime 793 

Soda 859 

Beryllium oxide 1053 

Manganese oxide 1 143 

The total value of calcium sulphate was 1793 (1000 for 
the sulphuric acid and 793 for the lime); to find the 
percentage amount of sulphuric acid in one molecule of 
calcium sxilphate, the proportion would read: 

Calcium sulphate : Sulphuric acid ;: 

1793 : 1000 :: 100 : X 

Similarly, the quantity of lime in calcium sulphate could 
be calculated. Many more examples of this kind were 
given by Richter, but it is evident from these that he 
should be regarded as the founder of mathematical 
chemistry. His experiments demonstrated, beyond ques- 
tion, that there is a definiteness of combination on the 
part of the ultimate particles of matter; otherwise it 
would not be possible to have salts of definite composi- 
tion formed by bringing together calculated amounts of 
base and acid. 

The question has been asked why the observations of 
Richter were not known to the chemists of Germany, 
England and France. Why were they tinknown to 
Dalton? The reason seems to be, that the whole subject 
was considered from a purely mathematical standpoint, 
and the men who were then laying the foundations of 
chemistry were averse to introducing mathematics into 
their subject. 



THE THEORIES OF CHEMISTRY. 49 

Berthollet, who followed Lavoisier as the leader of the 
French School, published statements which were antag- 
onistic to the Dalton doctrine. He argued that the 
affinities of elements one for another varied; and that 
this affinity, this force of attraction or attracting power, 
determined the question of what quantities should be 
present in the combination, so that there might not only 
be the ratio 1:1, but the ratio 1 : 1| or 1 :1| and so forth. 
In short, the elements combine with one another in 
indefinite quantities by weight. 

Proust, a fellow-countryman, took exception to Ber- 
thollet's declarations. By exact experiments he con- 
firmed Dalton's statement: that the elements combine 
in definite quantities by weight. Proust also noticed 
that when two elements combine with one another in more 
than one proportion by weight, the second combina- 
tion was also definite. This he observed before Dalton 
announced his law of multiple proportions. Proust 
failed, however, to observe that this second combination 
was a multiple of the first. In brief, he just missed the 
law of multiple proportions. He confirmed his observa- 
tions by numerous analyses, but he did not make deduc- 
tions, and the credit of having made the discovery of the 
law of multiple proportions belongs to Dalton. 

The controversy between Berthollet and Proust began 
in 1799, and extended over a number of years. It was 
conducted in a perfectly friendly way, with the final 
result that Proust won. He showed that Berthollet had 
been analyzing the hydrates of bodies. These, on stand- 
ing, gave up more or less moisture, and consequently the 
ratio between, for example, the oxygen of an oxide and 
its metal, varied from day to day — depending upon the 
water present. 



50 THE THEORIES OP CHEMISTRY. 

Shortly after this, in 1805, Gay-Lussac, with Httmboldt, 
contended that one volume of oxygen and two volitmes 
of hydrogen united to form two volumes of water. This 
discovery has already been referred to (page 38) and was 
one which certainly could be looked upon as a confirma- 
tion of the law of Dalton. Gay-Lussac met with difficul- 
ties in his attempt to apply his observations to the theory 
of Dalton, owing in part to the very confusing ideas 
which were entertained concerning the atom of Dalton. 
However, he pubHshed his results in these words: "gases 
combine chemically according to definite volumes." This 
was in 1808. He made a careful study of this law alone 
and in conjunction with Humboldt, and said that in the 
union of hydrogen and a halogen, one volume of hydrogen 
gas and one volume of chlorine gas combine to form two 
volumes of the halogen hydride. Hence, the halogen 
hydride occupied the space previously occupied by its 
constituents. This was an important deduction for the 
chemical atomic theory. It was concluded that as 
hydrogen and chlorine united in the ratio of their combin- 
ing weights, the ratio between the weights of equal volumes 
of these gases must be the same as that between the 
combining weights of these elementary gases, and that, 
consequently, according to Dalton, it could be assumed 
that the combining weight and the atomic weight were 
one and the same. The gas density would then be pro- 
portional to the atomic weight. Gay-Lussac and Hum- 
boldt were assuming that in equal volumes of different 
gases there were present an equal number of atoms — 
smallest particles. If 1000 atoms of hydrogen con- 
stituting one volume, united with 1000 atoms of chlorine, 
constituting one volume, there would result, according to 
these investigations, 1000 atoms of hydrogen chloride or 



THE THEORIES OF CHEMISTRY. 51 

two volvtmes. One volume of hydrogen chloride would, 
therefore, contain only half as many particles — or atoms 
— ^as a like volume of the elementary gases, a conclusion 
which they saw contradicted the laws relative to the 
constant physical deportment of the gases, which laws 
had previously been discovered by them. To solve this 
contradiction required almost a half century of chemical 
investigation. 

John Dalton, as previously noted, was not willing to 
accept any of these generalizations or deductions of the 
French School. Berzelius did accept them and used them 
in his magnificent work upon the atomic weights, to 
which reference will be made later. 

In 1811, Avogadro annoimced a law, given on page 40, 
that tmder similar conditions of temperattire and presstire, 
equal volumes of gases contain the same number of par- 
ticles, which need not of necessity be atoms. It was 
probably Berzelius who suggested the term atoms. 

This law, which seemed to offer an explanation of the 
difficulty which had been encountered by Gay-Lussac 
and others, did not meet with approval. There was 
apparently no need for it at that time, and it remained 
almost forgotten until 1850. The hypothesis of Avogadro 
is sometimes called the hypothesis of Ampere, since it 
was published by him, in 1814. 



During the opening years of the nineteenth century 
(1800-1810) it became evident that the science of chemis- 
try was, from that time on, to develop along several dis- 
tinct paths. These paths are not divergent, rather are 
they parallel. It not infrequently happened that workers 



52 THE THEORIES OF CHEMISTRY. 

in one were found among those actively concerned in the 
other lines of research. Discoveries made in one line 
were eagerly employed, wherever possible, in pursuing 
the work along other lines. 



Many chemists became interested in the atoms and in 
a determination of their relative weights. It soon became 
apparent to them, as this work progressed, that new 
methods must be found by which they were to determine 
these weights. As a consequence of this necessity, new 
working methods were discovered and descriptions of 
these will be found in this discussion of atomic weight 
determinations. 



Those engaged in the second line of inquiry sought to 
explain the possible constitution of chemical compounds. 
Many theories were proposed and a study of them is 
most interesting. 



A number were led to search for methods of experimenta- 
tion whereby the constitution of chemical substances could be 
determined, and this subject soon constituted itself a 
third line of research. 



In 1801, still another line of inquiry was begun which 
led to the amassing of theories and facts which finally 
culminated in the theory of electro-chemistry. The theories 
formulated to explain solution, because of their many 
points of contact with those of electro-chemistry, are 



THE THEORIES OF CHEMISTRY. 53 

included in this discussion. Electro-chemistry, the last 
subject to be considered, is thus placed because its greatest 
development has occurred during the closing years of the 
nineteenth century and the opening years of the present 
century. 



Determinations of the Relative Weights of the 

Atoms. 

Dalton's first table of atomic weights (1803) was both 
imperfect and incomplete. Thomas Thomson's estima- 
tions of atomic weights were even more faulty than 
Dalton's. In 1815, he wrote, *'all the chemical elements 
are aggregations or condensations of one and the same 
primordial substance, because (1) gas densities are mul- 
tiples of hydrogen, and (2) it appears that all atomic 
weights ought to be expressed in whole numbers, that is, 
multiples of the atomic weight of hydrogen, taken as a 
imit." This thought was not original with Thomson, 
although it appeared in his book. The author of it was 
an Englishman named Prout, who said that all atomic 
weights would be found to be multiples of imity. 

In 1818, Meinecke, a German, announced the same 
thought. Naturally, discussions followed. 

Berzelius and Turner proved Prout 's hypothesis to be 
incorrect. Turner's table of atomic weights, however, 
was incomplete. 

It was about this time that Berzelius began his mem- 
orable labors upon the atomic weight determinations. 
He had observed the efforts of Dalton and others, and 
was not pleased with the results obtained. He found 
occasion to question the purity of the materials used. 



54 THE THEORIES OF CHEMISTRY. 

Further, he was not satisfied with the standard that had 
been selected by John Dalton and by his adherers, 
"because," he said, ** hydrogen is an element with which 
comparatively few of the elements combine to form 
volatile compounds. Hydrogen unites directly with few 
of the elements. Most of the elements, however, combine 
with oxygen; and, therefore, it would be better for prac- 
tical purposes to make oxygen the unit of comparison, and 
to determine the ratio between hydrogen and oxygen." 
With this suggestion of Berzelius and the previous sugges- 
tion of Dalton, before the minds of chemists, there arose 
a controversy which has continued through many years. 
To most people this controversy is a matter of very little 
interest ; but to chemists, it is a matter of importance. 

The determination of atomic weights forms a most 
important field in chemical endeavor, and deserves as 
much consideration as other factors, namely, the physical 
properties of the elements, etc. The atomic weights are 
among the constants of nature and should be studied 
both from the point of theory, as well as from that of 
practical utility. 

One fact promulgated by John Dalton remains and, 
to-day, rises like Banquo's Ghost, not to disturb dreams, 
but to seriously harass wide-awake, thoughtful men. It 
is that the weight of the hydrogen atom is one, and this 
element with its unit weight is, or should he, the standard 
of comparison in deducing the atomic weights of other 
elements. 

Among those who had entered this field of investiga- 
tion was the mighty Nestor of the North — ^the honored 
and revered Berzelius. He made exhaustive analyses of 
numerous compounds, from which studies he drew definite 
conclusions, and gave the most complete list of atomic 



THE THEORIES OF CHEMISTRY. 55 

weights ever published (see 5th volume of Berzelius' 
' * Handbuch der Chemie ") . 

Berzelius declared that one of the simplest methods of 
determining atomic weights consists in weighing all 
those bodies as gases which can be gasified and comparing 
their specific gravities with one another. However, this 
method is only applicable to very few bodies, and if 
applicable, the restdt, to be accurate, requires a high 
degree of skill in manipulation and very careful attention 
to a host of difficulties which must be overcome. One 
such difficulty is the securing of absolutely piure gases, 
another is the weighiag of the gases, etc. And he adds, 
^'The commonest method to arrive at the atomic weights of 
elements is to combine them with oxygen, and to analyze 
the products with the greatest possible accuracy. To render 
the foimd atomic weights comparable, choose one as the 
unit, and with it compare all the others — just as the 
specific gravity of a substance is compared with water 
taken as the unit of comparison. Dalton chooses hydro- 
gen, because its atomic weight is the smallest of all 
the recorded values. I prefer to use the atom of oxygen 
as the standard because most compounds, with which 
chemistry is occupied, are oxides or compotmds of oxides. 
I assign to the atom of oxygen the value 100." 

Two masters in Israel, as may be seen, have given the 
units of comparison which in their respective judgments 
are best suited for the fixing of the atomic weights of the 
remaining elements. Dalton philosophized : "His inmost 
mental natiure and all its outward manifestations were, 
in the language of the German metaphysicians, emphati- 
cally subjective. ... In special or objective chemis- 
try, he has left absolutely no sign of his presence; no 
great monograph on an individual body and its com- 



56 THE THEORIES OF CHEMISTRY. 

potind; no memorable analysis of a substance deemed 
simple into yet simpler elements; no new element — ^no 
Neptime — added to the domain of chemistry." Professor 
Sedgwick wrote: ''From the hour he came from his 
mother's womb, the God of Nature had laid his hand 
upon his head, and had ordained him for the ministration 
of high philosophy." 

Berzelius experimented. He wrote, "I soon convinced 
myself by new experiments that Dalton's nimibers were 
wanting in that accuracy which was requisite for the 
practical application of his theory. I perceived if the 
light which had risen upon the whole science was to be 
propagated, the atomic weights of as large a number of 
elements as possible, and above all of the most commonly 
occurring ones, must be determined with the greatest 
accuracy attainable. Without work of this kind, no day 
could follow the morning dawn. This being the most 
important point for chemical research. . . . I devoted 
myself to it with restless activity. After work, laboring 
for a period extending over ten years. ... I was 
able to publish the atomic weights of about 2,000 simple 
and compound substances." 

And of this experimenter, it has been written (H. Rose) : 
**When a man, endowed with exceptional talents as an 
investigator, enriches every branch of his science with 
the most pregnant facts; distinguished himself equally 
in empirical and speculative research; grasps the whole 
subject in a philosophic spirit and arranges each detail 
systematically and clearly; gives out to the world a 
doctrinal system, critically sifted and put in as perfect a 
form as possible; and, lastly, when he proves himself a 
noble example of a practical and theoretical teacher, to a 
circle of pupils eager for knowledge, that man so fulfils 



THE THEORIES OF CHEMISTRY. 57 

the highest demands of his science, that he will continue 
to shine forth as a brilliant model for ages to come." 

That atomic weights were and are still needed, is 
evidenced by the attempts to prove or disprove Prout's 
hypothesis; and in the search for confirmations of the 
generaHzations of Mendelejeff and Lothar Meyer. Grant 
them, then, theoretical importance. Have they any real 
practical value? When a chrome ore is purchased on a 
percentage basis — and one of the contracting parties 
uses in the calculations Cr = 52.2, and the other Cr = 52, 
a difference arises. It may, then, be safely asserted that 
the determination of atomic weights, has both a theoret- 
ical and a practical value. Such determinations should, 
therefore, be made and made with all possible accuracy. 

Purity of material is the first essential in this work. 
It is also one of the most trying points in the problem. 
It demands that one should, by the most exacting 
methods, seek to eliminate every trace of extraneous 
substance. 

The material being pure, the next step is its analysis. 

"The accuracy of the analyses of Berzehus, being the 
admiration of the world, he was requested to make known 
his methods of procedure.'* Sebelien gives his answer in 
these words: -- 

"Try to find that method in analjrsis, in which the 
accuracy of the result will depend, to the least extent, on 
the skill of the operating chemist; and when this method 
has been selected, then consider what unavoidable condi- 
tions are present which may cause errors in the result, 
and ascertain v/hether they will increase or diminish the 
same. Thereafter make another determination, in which 
exactly the opposite effects only can be produced. If 
the result remains the same, the determination was 
correct." 



58 THE THEORIES OF CHEMISTRY. 

The analyses made, calculations follow. What shall 
be the unit of comparison? The hydrogen atom or the 
oxygen atom? This question has often been asked in 
the past; it is asked to-day. Down through the century 
which has elapsed since these standards were proposed, 
frequent changes have been made in their values, until at 
last with oxygen as the standard, it has received the 
value 16. This was believed to be its value compared 
with hydrogen =1. A brilliant array of eminent men 
(none of whom stands out more prominently or deserves 
more praise than our fellow-countryman, Edward W. 
Morley) devoted their services to determining the exact 
ratio existing between hydrogen and oxygen. After 
years of experimentation that ratio is H:0 :: 1:15.88; 
but if 0=16 then H = 1.0084. Whom does this affect? 
The theorist? Not at all — every analyst — every indi- 
vidual making chemical calculations is affected, or his 
work is; and it is largely because of the inconveniences 
to practical men, that in most recent times efforts have 
been made to establish a uniform basis. Three systems 
have come into use especially in the calculation of the 
results of organic bodies : 

1. H= 1 C=12 = 16 

2. H=/^'^^^| C = 12 = 16 

3. H= 1 C = 11.91 = 15.88 

To what extent would the molecular weight of any 
substance, for example, oxalic acid or of cane sugar be 
affected by these values? 



System 1. 


System 2. 


System 3. 


Oxalic acid: 126 


126.06 


125.10 


System 1. 


^System 2. 


System 3. 


Cane sugar: 342 


342.22 


339.60 



THE THEORIES OF CHEMISTRY. 59 

The question is answered by these two examples. In 
this uncertainty as to what should be done, the German 
Chemical Society several years ago appointed a com- 
mittee to examine into the question of a proper standard. 
That committee asked similar societies of other nations 
to imite with them in solving the problem. Three ques- 
tions were propounded. Two of them will be given here: 

1. Shall 0=16 be fixed as the future standard for the 
calciilation of atomic weights? 

2. Shall the atomic weights be given with so many 
decimals, that the last figure is certain within half a unit 
or what other procedure shall be adopted? 

The first question alone is of interest in this connection. 
Forty-nine replies were received. Forty advocated the 
oxygen standard (0=16); seven favored the hydrogen 
standard (H = 1), while two were undecided. This result 
was duly published and commented upon. The over- 
whelming vote for = 16 was hoped to be final — ^it has 
not been. It was argued that teachers and technical men 
had not been given the opportunity to express their 
opinions. This was then done both through the com- 
mittee of the German Chemical Society and through a 
committee of the Verein der deutschen Chemiker. While 
the questions were again being considered the German 
committee issued two atomic weight tables: 

The International Table 

= 16 and 11 = 1.008 
and The Didactic Table 

= 15.88 and li = I 

In the international table, about half of the recorded 
atomic weights are whole ntmibers or very close thereto, 



60 THE THEORIES OF CHEMISTRY. 

while in the didactic table there are about ten expressed 
by whole numbers. To the practical man, to any one 
using . these values frequently, the international table 
appeals strongly, because of its absence of decimals and 
consequent brevity. Since the result of the second vote 
has been published, there may be introduced here what 
appeared in the ''Berichte" in December, 1901. 

ForH = l = 16 

1 . Great International At. Wt. Com 7 40 

2 . Vote sent to Berlin, since first ballot 16 17 

3 . Through Volhard's Com 83 21 

106 78 

Vereine f or H 1 

Vereine f or O 4 

Inasmuch as four of five "Vereine" had voted for 
= 16, it was thought best to adopt the ''International 
Table of Atomic Weights." Perhaps abroad, more than 
in this country, there have arisen objections to this stand- 
ard. Some of the arguments for the retention of the 
standard (H=l) may be given. It has the advantage 
of being the original Daltonian standard; it is the most 
natural basis for atomic weights, as the hydrogen atom is 
the lightest atom known and because hydrogen is the 
standard in gas densities and for valence. If the oxygen 
standard is adopted, then all vapor densities must be 
changed. An approximate imit, like 1.008, is meaningless. 
To teachers the hydrogen standard appeals, because it is 
easily intelligible to beginners, whereas the oxygen stand- 
ard is somewhat difficult to explain. The physicists prefer 
the hydrogen standard because it approaches most nearly 
to an ideally perfect gas. " In the hydrogen thermometer, 
its nearly uniform rate of expansion is considered as well 



THE THEORIES OF CHEMISTRY. 61 

as the fact that at low temperatures it is the last gas, 
except helium, to assume the liquid state. The gradua- 
tion of the hydrogen thermometer is the ultimate stand- 
ard of reference in all except thermometry . . . this 
question of harmony between different branches of science 
is one which needs to be seriously considered. . . ." 

What can be said in favor of the other unit? Few 
elements can be directly compared with hydrogen; the 
latter has only been the theoretical standard, while the 
real basis in calculation has been oxygen. The familiar 
atomic weights are all in accord with this basis. The 
hydrogen scale is tmfamiliar and its adoption in its modem 
form would oblige chemists to drop old values and learn 
the new. 

Should not the opinions of the men who have devoted 
much of their time, energy and thought to the solution 
of this problem, have some weight? They of all others 
are alive to the existing difficulties — seen or imseen — 
arising from a selection of a imiform standard, and but 
two, Winkler and Mallet, of these men voted for the atom 
of H = l, as standard. Again, an atom of any other 
element could be chosen as the standard, for example, 
an atom of C=12. It is probable, from unpublished 
work, that hydrogen and oxygen would be fotmd to possess 
values very close to 1 and to 16 were the C standard 
selected. At present, however, consensus of opinion has 
declared in favor of the oxygen standard — = 16 with 
H= 1.0084. 



During the first years of the nineteenth century the 
workers in this field met with many difficulties. The 



62 THE THEORIES OF CHEMISTRY 

chemical atomic theory had not become an established 
fact, and there were times when it seemed as if it would 
be set. aside. Prout's hypothesis that all atomic weights 
woiild be found to be multiples of unity, met with very 
nearly general approval. As has been previously stated, 
Berzelius and Tiuner (about 1808) drew chemists away 
from this hypothesis and brought them back to the 
atomic theory. The work of Berzelius exerted a very 
great influence in more firmly establishing Dalton's 
hypothesis and in making his theory the basis of 
chemical science. 

Another difficulty confronted those engaged in the work 
of determining the weight of the atoms — the uncertainty 
which existed concerning the formulas for substances. 
Berzelius, in partictdar, pointed out the imsatisfactory 
condition pertaining to the formula for water, from which 
was to be obtained the correct relation existing between 
the atomic weight of hydrogen and that of oxygen. 
From both the analysis and S3mthesis of water, there 
was invariably obtained two volumes of hydrogen and 
one of oxygen. The proportion of hydrogen to oxygen 
by weight had been proven to be 1 : 8 — or the oxygen 
was eight times heavier than the hydrogen. In order 
to show these two facts, Berzelius wrote the formula 
for water H@, since he thought that oxygen was present 
in one to half of sixteen parts, or two of hydrogen to 
sixteen parts of oxygen; but this he could not prove. 
Other formiilas were being used for water; frequently 
two or three formulas were employed for one and the 
same substance. 

At this time Sir Htimphrey Davy brought forward a 
new thought. Davy had been sceptical concerning 
Dalton's rights as the originator of the atomic theory, 



THE THEORIES OF CHEMISTRY. 63 

and while he had later frankly recognized Dalton's service 
to the science of chemistry, he still refused to accept 
many of Dalton's conclusions. Davy was not ready to 
admit that Dalton had obtained atomic weights. He 
considered them to be the proportion numbers of the 
elements. 

In 1808, WoUaston had raised the question as to whether 
these weights were "atomic weights'* or were they not 
rather ''equivalent weights?'' 

Gay-Lussac likewise rejected the idea of atomic weights; 
he thought that only a ratio number was obtained, 

Berzelius believed the numbers obtained to be atomic 
weights. He worked alone. He appreciated the value 
of the discovery of Gay-Lussac's Law of Gas Voltmies 
and, guided by this law, drew his conclusions concerning 
the union of elements; he fotmd this law a valuable aid 
in determining the composition of many substances. 

A glance at the table of atomic weights, published in 
1818 by Berzelius, shows how reliable was his work when 
the values are compared with those of the present day. 
He used oxygen =100 as the standard, and when his 
values for atomic weights are reduced to the basis of 
0=16 (H@), carbon= 12.12, sulphur = 32.3, iron=109 
(from the formula Fe02). 

The chemists of that time, like the chemists of to-day, 
were thinking over this point of the determination of 
the relative atomic weights, and were making all manner 
of suggestions, were executing all sorts of experiments 
in the hope of discovering some method which would be 
generally applicable. 

The year 1819 brought two important discoveries, 
each in the field of physical chemistry. Both of these 
discoveries were valuable aids in the hands of Berzelius. 



64 THE THEORIES OF CHEMISTRY. 

The first was a contribution of Dulong and Petit, in which 
they stated the law of atomic heats; the second discovery 
was made by Mitscheriich, whose work was embodied 
in the law of isomorphism. 

Dulong and Petit found there was a simple relation 
between the combining weights and the physical proper- 
ties of the elements that is, their specific heat or their 
capacity for heat in the solid state. The atomic weights, 
therefore, were approximately inversely proportional to the 
specific heats, and as the result, the product oj these two 
values was found to he nearly the same for all elements. 
The atomic weight could be obtained, conversely, by 
dividing the constant (atomic heat) by the specific heat. 

It was found necessary to alter the so-called combining 
weights of some of the elements in order that they might 
harmonize with this law. As might be expected, there 
were objections raised, but to-day the changes are 
willingly accepted. <> 

The explanation of the fact in regard to the relation 
between combining weights and the physical properties 
of the elements is very simple. A few sentences from 
Lothar Meyer's work on theoretical chemistry will 
demonstrate this: 

"As the specific heat is the amoimt of heat required to 
raise the unit weight of the substance from zero degrees 
to one degree centigrade, this product represents the 
amount of heat required to raise the equivalent weight 
by one degree centigrade. The weight of the given ele- 
ment, which is heated one degree centigrade, is termed 
the thermic equivalent weight. If we regard this as the 
atomic weight, then the product of the atomic weight 
into the specific heat is the atomic heat, that is, the 
amoimt of heat taken up by one atom. It is clear that 



THE THEORIES OF CHEMISTRY. 65 

the atoms of the different elements have the same capacity 
for heat. The law may be simply expressed by saying 
that the atomic heats of all elements are approximately 
equal. This law can be applied without exception to 
all the malleable metals, to nearly all the brittle metals, 
to the majority of non-metals." 

The specific heat of a substance, then, is the quantity 
of heat which increases its mass one degree in temperature. 

Specific Heat X Atomic Weight = Atomic Heat 
H X A = AH 

Dulong and Petit determined the specific heats of copper, 
gold, iron, nickel, lead, bismuth, silicon, tin, zinc, cobalt 
and silver. They foimd the constant (AH) to be nearly 
6.25. 

Regnault determined the specific heat of a large number 
of elements, and obtained the constant 6.4. 

Berzehus multipHed the specific heat of lithium (0.94) 
by its atomic weight (7 — ^as he deduced it) and obtained 
the product 6.5. The same chemist multipHed the 
specific heat of sodium (0.293) by (23 — the value that he 
found for its atomic weight) and the product was 6.7. 
The specific heat of phosphorus 0.17 multiplied by 30.9 
gave 6.2 for a product. The mean of these constants 
was found to be 6.25. This method was a new and a 
decided aid in fixing the atomic values of the various 
elements. Berzelius altered, in consequence, a few of 
the results he had previously obtained. 

The observations of Dulong and Petit in regard to the 
relation existing between atomic weights and specific 
heats being correct, there is provided a very simple means 
of determining the atomic weights of elements when 
once their specific heats are determined, because it has 



66 THE THEORIES OP CHEMISTRY. 

been stated that if the constant 6.25 be divided by the 
specific heat, the atomic weight will result, so that this 
method gives a means of deciding which multiple of the 
combining number of an element is to be accepted as 
most probably expressing the atomic weight of that 
element. One illustration will suffice: 

Weigh a piece of metallic tin, oxidize it with nitric 
acid, convert it into the oxide of tin, and re-weigh it, 
the increase in weight represents oxygen, by means of 
a proportion a number is obtained for the tin : 

Weight of Oxygen : Weight of Tin : : Atomic Weight of Oxygen : X 

16 :X 

but if the proportion reads: 

Weight of Oxygen ; Weight of Tin : : 2 (Atomic Weight of Oxygen) : X 

32 :X 

then a different number results which is twice the first 
number. These results are the equivalent weights, but 
which is the atomic weight of the tin? It is at this point 
that the specific heat method is helpful. Weigh a 
quantity of tin (25 grams) heat that quantity to one 
himdred degrees centigrade, and when it has attained 
that temperature, suddenly throw it into water of a 
definite temperature. The heat of the tin passes to 
the water and raises the temperattire of the water, and 
the increase in temperature bears a definite relation to 
the weight of the tin. It gives its specific heat. The 
constant 6.25, divided by that specific heat, will give 
either the value of X obtained in the first proportion or 
that of X of the second proportion, and the number 
which the specific heat gives is the most probable atomic 
weight. 



THE THEORIES OF CHEMISTRY. 67 

The specific heats of forty-five soHd elements have 
been directly determined and their atomic heats are 
nearly all 6.25. Six of the elements have had their specific 
heats determined directly in the solid state, and their 
atomic heats have proved to be 5.5. And five elements 
have had their specific heats determined in the solid 
state, and have given an atomic heat of nearly 6.25. 
The atomic heats of the gaseous elements are all very 
doubtful and apparently they are variable. 

If fifty of the known elements yield this factor 6.25, 
it is evident that the law of Dulong and Petit is of some 
consequence; it cannot be the final method in determin- 
ing the atomic weight of any element, but it can be of 
great assistance. 

Attention, however, should be called to the exceptions 
to this law. They are beryllium, boron, carbon and 
silicon. These are important elements, but Pattison 
Muir thinks, in regard to beryllitmi, that too much value 
ought not be to given to its specific heat in the charts 
and tables, because the material that was used for its 
determination seems to have been impure. 

He refers to the work of the Swedish chemists (Neilson 
and Pettersson) who conducted experiments on the specific 
heat of metallic beryllium containing known quantities 
of berryllium oxide, iron oxide and siHca. They also 
determined the specific heat of pure berryllium oxide, and 
the specific heats of iron oxide and silica being known, 
they deducted values for these impurities from the specific 
heat of their mixtiu-e, and found the specific heat of beryl- 
lium was 0.407. The same chemists made a second 
series of determinations with a sample of the metal 
containing only about five per cent of beryllium and 



68 THE THEORIES OF CHEMISTRY. 

ferric oxide, and obtained values for the specific heat 
varying from 0.397 to 0.505; hence these chemists con- 
cluded that the atomic weight of beryllium should be 
changed from 9.1 to 13.65. These experiments on beryl- 
lium are not conclusive. Will these impurities have the 
same specific heat in the mixture as they possess when 
alone? 

All the more recent work on the atomic weight of 
beryllium, for example, the determination of it by one 
of the cryoscopic methods, gives 9.1 as its atomic weight. 
It is a bivalent metal and not trivalent, as it would be 
with the higher atomic weight. 

Concerning the other three exceptions to the law of 
Dulong and Petit, Weber foimd the specific heats of car- 
bon, boron and silicon increasing rapidly with rise of 
temperature, but at high temperatures the rate of increase 
was much smaller. 

From the foregoing facts, it appears that the specific 
heat of every elementary atom is not a constant number 
but varies with the temperature, and that the relation 
between the variation of specific heat and that of tem- 
perature differs for each element. Hermann Kopp says 
of these three elements (boron, carbon and silicon) : 

"The atoms of these elements are probably built up 
of simpler parts, have a grained structure, and at high 
temperature, they are composed of a smaller number of 
these parts than they are at a lower temperature. Heat 
added at low temperatures is consumed in separating 
these atomic groups." 

Muir adds: "the facts of spectroscopy seem to point to 
the existence of more complex structure in the non- 
metaUic than in the metallic molecules; that allotropy 



THE THEORIES OF CHEMISTRY. 69 

occurs distinctly among non-metals; that the moleciiles 
of the five metallic elements, whose vapor densities have 
been determined, are monatomic; that the atomic heat 
of telluriiim, a metal like a non-metal, belonging to the 
group of oxygen, is 6 ; that of the less metallic selenium 5.8 ; 
that of the decidedly non-metal sulphur 5.5; and the 
typical non-metal, oxygen, not more than four; and, 
finally, that the molecular structures of oxygen, selenium 
and sulphur vapors are more complex than the structure 
of tellurium vapor." He concludes, *'as silicon, carbon 
and boron are distinctly non-metallic, these facts lend 
support to the idea that part of the heat added to the 
carbon, the boron, and the silicon, at low temperatures 
is spent in separating the complex molecule groups 
into their constituent parts, rather than in separating 
the hypothetical complex atoms of the elements into 
smaller atoms." This is a matter which should not 
be lost sight of in considering the variations which have 
been discussed. 



Mitscherlich's law of isomorphism, the second dis- 
covery made in 1819, also gave another means of deter- 
mining the atomic weights. It was a less general and not 
so certain a means as the preceding discovery of Dulong 
and Petit. 

This law was published in 1821, and in this form: 
"Equal nimiber of atoms in a molecule, similarly combined, 
exhibit the same crystalline form; identity of crystalline 
form is independent of the chemical nature of the atoms, 
and is conditioned only by the number and configiuration 
of the atoms. ' ' Accordingly, the quantities of the elements 



70 THE THEORIES OF CHEMISTRY. 

occurring in isomorphous mixtures, would be to each 
other as their atomic weights. This fact makes it pos- 
sible to establish a value for the atomic weight. There 
were two drawbacks to the law of Mitscherlich: 

(1) Similar chemical bodies do not necessarily have 

identical crystalline forms, and 

(2) Unlike chemical bodies may crystallize in identi- 

cal forms. 

The law could only be applied in cases where substances 
were strictly isomorphous (in cases where overgrowths 
occur). For example, if zinc sulphate will grow over 
copper sulphate crystals and the true form of the copper 
sulphate be retained then these two sulphates are 
strictly isomorphous. Isomorphism frequently occurs 
where there is no similarity in chemical composition; 
but overgrowths only take place where the composition 
is analogous. 

Pattison Muir has written at some length, in order to 
show to what extent this rule of Mitscherlich is applicable. 

Mitscherlich introduced the terms dimorphous, tri- 
morphous, polymorphous. Many examples of the 
phenomena to which these terms were applied are now 
known. For example, calcium carbonate crystallizes in 
hexagonal form as calcspar and in rhombic form as 
arragonite. 

The application of the law of Mitscherlich and of the 
law of Dulong and Petit to one example (determination 
of the atomic weight of iron) will suffice for illustration: 
The atomic weight of the element chromium is 52.4. 
The green oxide of chromium shows the same crystalline 
form as ferric oxide; these two oxides should, therefore, 
be represented by similar formulas. The atomic weight 



THE THEORIES OF CHEMISTRY. 71 

of chromium being 52.4, and oxygen 16, the simplest 
formula that can be given to the green oxide of chromium 
is Cr203, and the formula for ferric oxide is, therefore, 
Fe203. If Fe203 is the correct formula for ferric oxide, 
it follows that two atoms of chromium are replaced, in 
one reacting weight of the oxide, by two atoms of iron; 
and the atomic weight of iron would be one-half the total 
v/eight of iron in the oxide — or 56. The specific heat of 
iron multiplied by 56 gives the constant 6.55, therefore, 
56 is almost certainly the atomic weight of iron, and 
the formula for each oxide is correct. 

The study of the crystalline form of substances has been 
an aid in the determinations of atomic weights. Before 
more, extended and precise knowledge of the connections 
between crystalline form and chemical constitution is 
obtained, this method of determining the atomic weights 
of elements may only be applied tentatively and in a very 
few cases. The method is of use in suggesting possible 
lines of research. It is not improbable that the crystal- 
line form of a substance is connected with the internal 
structure of the molecules. 

This law has received considerable attention since 
1819. Chemists and physicists, alike, have been anxious 
to make it applicable and trustworthy; so that it has 
been modified and remodified. It was made to read: 
*'Isomorphous bodies have either similar chemical com- 
position or the quantities of the elements in the compound 
vary but slightly." 

Hermann Kopp, one of the earliest of the physical 
chemists, sought to simplify the existing chemical laws, 
and he modified this law of Mitscherlich to read, "iso- 
morphism will exist only when bodies are capable of grow- 
ing in a new solution," 



72 THE THEORIES OF CHEMISTRY. 

Notwithstanding the attempts to modify this law, so 
as to accord with facts, it is of Httle assistance for the 
purpose for which it was suggested. It has been instru- 
mental in determining the atomic weight of iron and of 
gallitmi. 

Following the annotmcement of these two discoveries, 
Berzelius did not, at first, think that a revision of his 
atomic weights was necessary; but in 1826, he published 
a new table in which many of the values were halved — 
some were entirely changed. In this tabje Berzelius, 
for the first time, gives nitrogen and chlorine as simple 
substances. 

The great difficulty still remained: What was the cor- 
rect formula for water and the correct atomic weight 
for oxygen? 

Gerhardt believed that H2O was the correct formula 
for water, since it consisted of two voltimes of hydrogen 
and one of oxygen, and eight times as much oxygen by 
weight as hydrogen; Berzelius, as has been seen, expressed 
the same thought by the formula H0. Neither of these 
men could prove their speculations. In 1827, Dumas 
made a discovery which solved the problem. He dis- 
covered the method whereby the vapor density of sub- 
stances could be determined. A definite quantity of a 
substance (in this case water) was converted into vapor, 
and this vapor compared with an equal volume of a stand- 
ard, hydrogen, or air, or any other standard, but hydrogen 
was preferred. The weight of water vapor was thus 
found to be nine times heavier than one volume of hydro- 
gen. The molecular weight of water would, therefore, 
be eighteen, and if in these eighteen parts by weight 
of water, there are two parts by weight of hydrogen, sixteen 
should represent the weight of the other constituent of 
water, namely, oxygen. 



THE THEORIES OF CHEMISTRY. 73 

Dtimas* method proved to be a valuable aid in deter- 
mining certain atomic weights, but it was limited for two 
reasons: (1) all substances could not be converted into 
the vapor form, (2) chemists had not yet reached a proper 
understanding of the meaning of the law of Avogadro. 
When Avogadro's law was correctly interpreted, the 
second limitation was removed, and the method of Dimias 
became exceedingly helpful; it continued to be restricted, 
however, by the fact that not all substances could be 
vaporized. 

Dumas in applying his method to atomic weight 
determinations was led into error. Berzelius retained 
his numbers. A comparison made of the atomic weights 
of Berzelius and of Dumas with those of the present day, 
prove that Berzelius was justified in considering his 
determinations correct. 

About this time, Faraday had succeeded in decom- 
posing water and several other compoimds with the aid 
of the electric current, and he said ''that equivalent 
amounts of hydrogen or metal were separated at the 
negative pole, and the corresponding quantities of 
oxygen or chlorine at the positive." In a determina- 
tion of "electro-chemical equivalents" Faraday saw a 
sure secondary means for fixing doubtful atomic weights. 

During the passing years there had been growing a 
confusion of ideas concerning the terms "equivalent," 
"atom," "molecule," but the time was not yet ripe 
for a clear cut exposition of these terms. 



In 1831, Neimiann published determinations of the 
specific heats of various solid compounds, chiefly of 



74 THE THEORIES OF CHEMISTRY. 

naturally occurring minerals, and deduced the general 
statement '*that the amoimts of similar chemical com- 
pounds possess equal specific heats." It means this: — 
that the specific heat of free atoms is retained by them 
when they enter into chemical combination. Will this 
statement of Neumann be of any service? What are 
chemists trying to do? They (Berzelius, Dimias, Ger- 
hardt, Laurent and others) are endeavoring to find 
better methods by which to determine the relative weights 
of the atoms of the various elements. Dumas' vapor 
density method was inadequate. There were exceptions 
to the specific heat method of Dulong and Petit. 

If, then, Neumann could get the specific heat of a 
molecule (a), and, if the specific heat of one of the 
constituents (b) was known, the specific heat of the 
other constituent could be obtained by subtracting (b) 
from (a). 

Neumann hoped that his observation might prove 
valuable in indirectly determining the atomic weights 
of certain elements, for example, chlorine — (l) the 
molecular or specific heats of metallic halides are: 
RC1=12.8, RCl2 = 8.5, RBr=13.9, RI = 17.4, Rl2 = 
19.4; (2) the atomic heat of each of the metals 
(R) is about 6.4; (3) the atomic heat of solid bromine 
and iodine is 6.6; (4) the chlorides, bromides and iodides 
are chemically analogous; (5) the moleciilar heats of 
analogous salts are nearly the same; it was, therefore, 
concluded that the atomic heat of solid chlorine was 
6.4, this divided by the specific heat would give the 
atomic weight by an indirect method. 

This method was applied to the determination of the 
atomic weights of fluorine, nitrogen, oxygen and a nimiber 
of other elements, and gave results which were astonish- 



THE THEORIES OF CHEMISTRY. 75 

ingly close to the numbers obtained for these elements 
by other methods, so that Neimiann's method seemed 
satisfactory. But, as time rolled on, exceptions were 
foimd; so that the method was eventually abandoned. 



Soon after the fourth decade of the nineteenth century 
began, Gerhardt and Laurent endeavored to bring about 
uniformity in the use of the chemical terms ''equivalent" 
and "atomic weight." At this time it was a question 
of much concern what atomic weights should be ascribed 
to the elements, and what atomic (i.e., molecular) weights 
should be ascribed to compotmds. Gerhardt's explanation 
was obscure. Laurent gave to his explanation greater 
clearness. Laiu-ent based his deductions upon Avogadro's 
hypothesis and thus brought this law again before chem- 
ists. He, further, distinguished between "atoms" and 
"molecules," defining " mo/^cw/^5 as the smallest particles 
of matter which could exist free or uncombined and which 
were absolutely necessary in order to bring about chemical 
action. Atoms were the smallest particles of an element 
which could occirr in chemical combination." "Equiva- 
lents" were the quantities by which elements replaced 
one another. He further showed that the so-called 
"equivalent weights of elements" must be regarded as 
their "atomic weights," and the ** equivalents of com- 
pounds" as their "molecular weights." 

This work of Gerhardt and Laxirent did not find imme- 
diate acceptance with chemists. Stronger proof was 
required that atomic weights should be applied to elements 
and molecular weights to compounds, than that given 
by these two chemists. 



76 THE THEORIES OF CHEMISTRY. 

Gerhardt was using H2O as the formula for water, 
which coincided in principle with the formula used by 
Berzelius, and with the deductions made and based upon 
Dumas' vapor density of water; but other chemists were 
using different formulas. It was dtiring the fifth decade 
of the nineteenth century, that chemists most keenly 
appreciated the fact that they were using formulas which 
scarcely expressed the truth. The confusion arising from 
the use of different formulas for one and the same sub- 
stance became very great. 

How, then, could the atomic weights obtained be cor- 
rect? Surely, this matter reqiiired speedy settlement, and 
it came in 1858. 

It was in 1811, that a man named Avogadro, a physicist, 
made a statement frequently referred to in this text 
which was, ''that in equal volimies of all bodies in the 
gaseous state there is an equal nimiber of particles.'* 
It was Berzelius, who, after some experimental study and 
reflection upon this statement, wrote the proposition of 
Avogadro to read as follows: "that in equal volumes 
of bodies in the gaseous state there is contained an equal 
number of atoms." 

The confusion which prevailed in the use of formulas 
and the desire to find some sure and common ground 
upon which to stand in this matter, led, in 1860, to a 
congress of the chemists and physicists of Europe. This 
congress was held at Carlsruhe. It was called on the part 
of the chemists of Europe at a time when the people of 
the United States were preparing to enter upon one of 
the most critical periods in the history of America. More 
than one himdred of those who had been invited accepted 
and appeared at Carlsruhe. Many of the chieftains of 
that time were missing, others after a brief stay departed. 



THE THEORIES OF CHEMISTRY. 77 

However, it may be said that in the years which have 
since passed, there has never been a more brilliant gather- 
ing of chemists an3rwhere in the world. 

During the congress, side issues so filled the days that 
the main subject for discussion was never reached. 

There was, however, one address delivered by an Italian, 
named Cannizzaro, which appealed to every one present. 
Dumas had delivered a lengthy address, in which he 
developed the thought that chemistry consisted of two 
distinct sciences — the inorganic and the organic. This 
address was followed by an impromptu speech by Can- 
nizzaro. He emphasized the fact that while organic 
chemistry had grown marvelously, and while it had been 
using other atomic weights and had been following other 
rules than those which were applied in inorganic chem- 
istry, yet the science of chemistry was -unitary in its 
character. 

It is needless to say that Cannizzaro won the sympathy 
of the entire audience, which was expressed in the addresses 
later delivered by Hermann Kopp and Otto Erdmann, 
who said, among other things, that chemists must seek 
to bring together facts that have been observed in their 
discoveries and to imite them in one common plan. 

This was in 1860. Lothar Meyer was present at that 
gathering. He was at Carlsruhe as a yoimg man, and 
he wrote that he went from this congress in a very unhappy 
frame of mind. By the merest accident, however, there 
fell into his hands a letter which Cannizzaro, in 1858, 
had written to Professor Lucca. In this letter, Can- 
nizzaro imdertook to set forth the methods by which 
he successfully proved to his students the correctness 
and the great importance of the law of Avogadro, which 
had been enunciated early in 1811. Lothar Meyer 



78 THE THEORiES OF CttEMISTRY. 

perused this letter on his way home from this congress, 
the scales fell from his eyes and he saw more clearly than 
he had ever seen before, how, by following the theory 
of AviDgadro, with the modifications put upon it by 
Cannizzaro, it was possible for chemists to arrive at 
definite conclusions concerning molecular formulas. The 
formula established, the correct atomic weights of the 
constituent elements might then be determined. 

That letter of Cannizzaro has since been published 
in ItaHan and, later, at the instance of Meyer, it was 
translated by one of his Italian students. It has been 
annotated and given out as one of the classics of the 
exact sciences in the series published by Ostwald. This 
book should be read because it is not only instructive 
but exceedingly helpful. 

Cannizzaro insisted that the generalization of Avo- 
gadro, ** equal volumes of gases contain an equal number 
of molecviles," afforded a direct method of determining 
the relative weights of the molecules of many elements 
and compounds, without any knowledge of the com- 
positions, or the reactions, of these molecules. "All 
that need be done," Cannizzaro said, *'is to determine 
the densities of the substances in the gaseous state, and 
express the experimental results in terms of the standard 
element, which is hydrogen." 

Avogadro had taken the molecular weight of hydrogen 
as imity. Cannizzaro said, ** Instead of taking for your 
unit the weight of an entire molecule of hydrogen, take 
rather the half of this weight, that is to say, the quantity 
of hydrogen contained in the moleciile of hydrochloric 
acid." This change of units brought home to chemists 
the principle they had been seeking for half a century; 
it put into their hands the knowledge they required 
and had overlooked. The following table of relative 



THE THEORIES OF CHEMISTRY. 



79 



densities of gaseous elements and compounds, and molecu- 
lar weights of these substances, is taken from Cannizzaro's 
memoir published in 1858. 



Name of substance. 


Densities 
or weights of a gaseous 
volume, referred to the 
weight of a volume of hy- 
drogen =1; or molecular 
weights referred to the 
weight of a molecule of 
hydrogen as unity. 


Densities 
referred to that of hy- 
drogen taken as 2 ; or 
molecular weights re- 
ferred to the weight 
of a semi-molecule of 
hydrogen as unity. 


Hydrogen 


1 


2 


Ordinary oxygen 


16 


32 


Electrified oxygen 


64 


128 


Sulphur under 1000°... 


96 


192 


Sulphur over 1000° 


32 


64 


Chlorine 


35.5 


71 


Bromine 


80 


160 


Arsenic 


150 


300 


Mercury 


100 


200 


Water 


9 


18 


Hydrochloric acid 


18.25 


36.5 


Acetic acid 


30 


60 



Since 1858, the determination of the molecular weight 
of homogeneous gaseous, or gasifiable substances has 
consisted in determining the density of that substance 
in the state of a gas. The restilt gives the number of 
times heavier a volimie of that gas is than an equal 
voltime of hydrogen, referred to the weight of a molecule 
of hydrogen =2, or referred to the weight of a semi-mole- 
cule of hydrogen =tmity. 

Cannizzaro taught chemists how to express the com- 
positions of molecules. He said, " If the substance cannot 
be decomposed, it is necessary to conclude that the mole- 
cules of it consist of one substance throughout. If the 
substance is a compoimd it is analysed, and the constant 
weight-relations of its constituents determined. The 
molecular weight is then divided into parts proportional 



80 



THE THEORIES OF CHEMISTRY. 



to the relative weights of the components, the quantities 
of the elements thus obtained in the molecule of the 
compound, are referred to the same unit as is used for 
the expression of all molecular weights." Cannizzaro 
illustrated this statement by a table of which the following 
is a portion : 

Weight Weights 

of a volume, or of the constitution of a volume, or 

Name of molecular weight a molecule, the sum being referred 

substance referred to the to the weight of a semi-molecule of 

weight of a semi- hydrogen as unity, 
molecule of hy- 
drogen as unity. 

Hydrogen 2 2 

Ordinary oxygen .. . 32 32 

Phosphorus 124 124 

Hydrochloric acid.. 36.5 35.5 chlorine + 1 hydrogen 

Water 18 16 oxygen + 2 hydrogen 

Calomel 235 .5 35.5 chlorine + 200 mercury 

Corrosive sublimate 271 71 chlorine + 200 mercury 

Carbon monoxide.. 28 16 oxygen +12 carbon 

Carbonic acid 44 32 oxygen +12 carbon 

Alcohol 46 6 hydrogen + 16 oxygen + 

24 carbon 

Ether 74 10 hydrogen + 16 oxygen + 

48 carbon 



Tables similar to this were arranged by Cannizzaro 
for several compounds of the same element. His table 
for several of the compotmds of chlorine follows: 

Weights of chlorine in the 
Name of compound containing chlorine. molecules referred to the 

weight of a semi-molecule 
of hydrogen as unity. 

Hydrochloric acid 35 . 5 = 35 . 5 

Chlorine 71 =35.5X2 

Mercuric chloride 71 =35.5X2 

Arsenious chloride 106 . 5 = 35 . 5 X3 

Stannous chloride 71 =35.5X2 

Stannic chloride 142 =35 . 5 X4 

etc., etc =35.5XN 

Atomic weight of chlorine = 35.5 



the theories op chemistry. 81 

Cannizzaro's Law of Atoms. 

Referring to this table and others like it, Cannizzaro 
said, "By comparing the different quantities of one and 
the same element, which are contained either in the 
molecule of the free element, or in the molecules of its 
compoimds, the following law stands out in relief: The 
different weights of one and the same element contained 
in the various molecules are always whole multiples of 
one quantity, which is justly called the atom, because 
it invariably enters the compoimds without division." 

The application of the hypothesis of Avogadro to facts 
established by experiment concerning the volume-relations 
of various elements and compoimds, led back to the con- 
ception of the atom. 

The term atom is now applied to each of those smallest 
fractions of a molecule which are produced and again 
combined when molecules interact. 

The weight of hydrogen in a molecule of hydrochloric 
acid gas is taken as unity, and it was agreed to call this the 
atomic weight of hydrogen. Then the weight of hydrogen 
in a molecule of that element is twice unity — ^in other 
words, the molecular weight of hydrogen is 2. 

It is very essential that the student should understand 
clearly the meaning of the assertion that the molecular 
weight of hydrogen is 2. A certain volume of hydro- 
chloric acid gas contains one part by weight (say, one 
gram) of hydrogen; an equal volume of hydrogen gas 
contains twice as much (two grams) hydrogen. Equal 
volumes contain (by hypothesis) an equal number of 
molecules; therefore, a molecule of hydrogen weighs twice 
as much as the hydrogen in a molecule of hydrochloric 
acid gas, that is, twice as much as the unit weight, twice 



82 THE THEORIES OF CHEMISTRY. 

as much as the atomic weight, of hydrogen ; in other words, 
the molecular weight of hydrogen is 2. 



The Molecule and the Atom. 

It was now made possible to form a clear notion of the 
expressions, molecule of a homogeneous gas, and an atom 
of an element. Avogadro said that the molecules of a 
gas are **the particles which are at such a distance from 
each other that they cannot exercise their usual action." 
In more recent times, Clerk Maxwell thus describes the 
molecule of a gas: "A gaseous molecule is that minute 
portion of a substance which moves about as a whole, 
so that its parts, if it has any, do not part company 
during the motion of or agitation of the gas." The 
molecule of a substance is sometimes described as the 
smallest mass of it which exhibits the properties which 
are characteristic of that substance. This description 
is often stated more briefly thus : a molecule is the smallest 
mass of a substance which can exist in the free state. 

The student must think of a gas as composed of a vast 
number of exceedingly minute particles, each of which 
has the properties which mark off the gas from all other 
substances; and consider these particles as moving about, 
sometimes colliding, sometimes traveling freely, but not 
breaking up (under ordinary conditions) into portions. 
The possibility of the occurrence of collisions between 
molecules of different gases must also be pictured. These 
collisions entail the shattering of molecules into parts; 
the student must see these parts — these atoms, rearrang- 
ing themselves and forming new molecules. The atom 
of an element is the smallest weight of it which enters 



THE THEORIES OF CHEMISTRY. 83 

into the composition of various molecules, referred to 
the smallest weight of hydrogen, as unity, which takes 
part in the formation of molecules. 

The term molecule is applied to elements and com- 
pounds; the term atom is applied only to elements. The 
molecule is generally conceived to be a collocation of 
atoms; although there are cases where the molecule and 
the atom of an element are identical. The atom is the 
ultimate particle wherewith the chemist is concerned 
at present. Chemistry neither asserts nor denies the 
possibility of dividing the atom; it affirms only that if 
an atom were to be divided, the properties of the parts 
would not differ from those of the whole. 

The hypothesis of Avogadro does not make any asser- 
tions regarding the relative sizes of molecules. 

Order was thus brought out of chaos by Cannizzaro's 
interpretation of Avogadro's hypothesis. Chemists now 
possessed a means whereby formtdas for chemical sub- 
stances might be fixed; and from which the weights of 
the constituent atoms might be deduced. 

Several theories had been proposed, before 1860, to 
explain chemical constitution. These theories, to some 
extent at least, prevented scientists from accepting Can- 
nizzaro's explanation of Avogadro's hypothesis. Meyer, 
in his "Modem Theories of Chemistry," remarks of these 
theories : "they came into shape slowly, and were adopted 
slowly." In discussing Gerhardt's theory of types, 
Meyer comments on the fact that in the four volumes 
of his "Treatise on Organic Chemistry," Gerhardt 
confines his description of the type theory to not over 
one hundred pages at the close of the last volume, 
and does not use the theory in elucidating the con- 
stitution of a single compound. Gerhardt was ques- 



84 THE THEORIES OP CHEMISTRY. 

tioned on this subject and confessed to a fear that his 
book woiild not be purchased if he had done otherwise. 
Meyer adds: "It is well for chemists, if this is really 
the condition of affairs, to turn away from fiu-ther dis- 
cussion of these various theories, and rettun to the 
determination of atomic weights; to make such investi- 
gations as will enable them to deduce correct formulas 
for the compoimds being used, and to refer to the 
future, the arrangement of these formulas in accordance 
with the light which may later be discovered." 

Half a century after the promulgation of Avogadro's 
hypothesis, the rapid development of organic chemistry, 
the knowledge of the density of gases, made necessary 
the logical and universal application of this hypothesis. 

Clausius, further, recognized the necessity for this 
hypothesis on purely theoretical grounds arising from the 
mechanical theory of heat. Avogadro adhered to the 
doctrine of the material natiu-e of heat, and, therefore, 
explained the expansion of gases by the repulsion exerted 
between the layers of heat siurounding the individual 
molecules. The hypothesis offers no means of explaining 
why the expansive force and its changes with the temper- 
ature, and with the change of volume, should be the same 
for all gases merely under these conditions. But it appears 
to be the only possible assimiption if certain general 
considerations are taken into account. 

By the aid of Maxv/ell, the relative frequency of any 
one of the many possible values of molecular velocity can 
be calculated. This calculation proves that in a mixture 
of several gases, the probability of a value for the energy 
of the molecules of a particular gas is just as great as 
for the molecules of any other gas. In a mixture of 
several gases, the molecules of one gas may possess double, 



THE THEORIES OF CHEMISTRY. 85 

treble, half or third the energy of the others, but, the 
number of molecules in equal volumes of different gases 
must, according to the law of probabilities, be equal 
under similar conditions of temperature and pressure. 
In this way Avogadro's hypothesis attains to the same 
degree of probability which the kinetic theory of gases 
has attained. Avogadro's hypothesis must remain indis- 
pensable, whatever views should be held regarding the 
nature of the gaseous state. Important objections have 
never been raised to this hypothesis; whenever it has been 
discussed, the question has been merely concerning the 
convenience of explaining the hypothesis — ^its correctness 
has never been disputed. 

The study of chemical reactions for the determination 
of the molecular weights, without the aid of Avogadro's 
hypothesis, is very deceptive and thoroughly inadequate 
as has been seen from the fact that molecular formulas 
and their corresponding molecular weights determined 
prior to the correct explanation of this hypothesis were 
universally regarded as imsatisfactory. 

It is not possible to determine the absolute magnitude 
of the molecular weights by means of the hypothesis, 
but the relative magnitudes of the molecular weights of 
all bodies, the density of which in the gaseous state has 
been measured, may be determined, and the molecular 
weights are proportionate to the densities. 

The chemists, who have exploited this hypothesis, 
agree that the weight of a molecule of hydrogen, which 
according to a previous statement contains at least two 
atoms, shall be represented as equal to two, but as Avo- 
gadro took the molecule and not the atom as tmity, 
the numbers at present used for the molecular weights 
are double those he proposed. 



86 THE THEORIES OF CHEMISTRY. 

This hypothesis in the form which it assumed after 
its modification and development by Cannizzaro, remains 
to-day and serves, as has been indicated, as the means 
for the deduction of molecular and atomic weights. 



In concluding the subject of atomic weight determina- 
tions, certain more recent considerations must be added. 
Front's hypothesis, previously referred to, according to 
which the atomic weights of all elements were to be 
regarded as simple multiples of the atomic weight of 
hydrogen, served for a long time to stir up strife and 
threatened the rational development of the atomic theory; 
that such a catastrophe did not occur is owing to the 
labors of the most eminent chemists of that time. 

During recent years, Front's hypothesis has again re- 
ceived attention. Dobereiner, L. Gmelin, Fettenkofer, 
Dumas, Odling and others observed that chemically 
analogous elements had atomic weights very nearly 
similar or which differed from each other by a definite 
increment. This fact gave rise to much spectdation. 
Recently, an attempt has been made to systematically 
classify the elements in such a manner, that the con- 
nection between the natures of elements and their atomic 
weights would be apparent. 

In 1864, Newlands and, a little later, Lothar Meyer, 
in Germany, independently arranged a number of the 
elements according to the magnitudes of their atomic weights^ 
and observed that while those following each other showed 
no great similarity in properties, yet, after the lapse of a 
certain "period" the chemical and physical behavior 
of the elements repeated the behavior of those in the 



THE THEORIES OF CHEMISTRY. 87 

preceding period. The elements resembling each other 
were placed in ''groups'' or ''natural families.'' ' The 
"periods" contain the elements having atomic weights 
between those of two adjacent elements in a "group. " 

After 1869, these beginnings were extended and per- 
fected by Mendelejeff and Lothar Meyer. At this time 
there existed greater accuracy in the atomic weights. 
Mendelejeff attempted to classify all of the elements 
according to the magnitude of their atomic weights. 
He showed that elements which were chemically similar 
(in groups or families) followed one another in regular 
periods. Elements were thus brought together in a 
"natural system," much, however, was arbitrary because 
of inaccuracy of the atomic numbers. The fimdamental 
idea developed by these investigations, namely, "that 
the elements arrange themselves on the one hand into 
periods, and on the other into natural families, and that 
all their properties are periodic functions of their atomic 
weights," has been strengthened by further investi- 
gations. 

By thus systematizing the elements it has been possible 
to make certain deductions, for example, definite values 
could be assigned to elements whose atomic weights were 
uncertain, since every element has a place of its own in 
the system; and as a consequence of this fact the atomic 
weight of molybdenum was corrected. Again, there 
were gaps in the system. Some of these have been filled 
by newly discovered elements, and these elements were 
predicted together with their approximate atomic weights 
and some of their properties, for example, germanium, 
scandium, etc. 

One result of this classification was to demonstrate 
the fact that physical and chemical properties of all 



88 



THE THEORIES OF CHEMISTRY. 



elements show a periodic dependence upon their atomic 
weights. The discovery of the cause of this is yet to be 
made. Some of the scientists have returned to the idea 
of a simpler element for each group — a reappearance of 
Front's hypothesis. 

Crookes has again brought up this subject. After 
making observations on the phosphorescence spectra 
of the yttrium earths, he claims that elements are not 
simple substances but are to be considered as compounds 







Group 


Group 
2. 


Group 
3. 


Group 
4. 


Group 
5. 


Group 
6. 


Group 
7. 


Group 
8. 




- 


RH 
R2O 


RH2 
RO(R202) 


RH3 
R2O3 


RH4 

R02(R204) 


RHs 
R2OJ 


RH2 

R03(R206) 


RH 
R2O7 


R04(R208) 


Periods 




H 


















He 


Li 


Be 


B 


c 


N 





F 






Ne 


Na 


Mg 


Al 


Si 


P 


s 


01 






A 


K 


Ca 


Sc 


Ti 


V 


Cr 


Mn 


Fe Ni Co 






Cu 


Zn 


Ga 


Ge 


As 


Se 


Br 






Kr 


Rb 


Sr 


Y 


Zr 


Cb 


Mo 


— 


RuRhPd 






Ag 


Cd 


In 


Sn 


Sb 


Te 


I 






Xe 


Cs 


Ba 


La 


Ce 


(Nd,Pr) 




— 


— — — 












Ya 





Ta 


w 





Os Ir Pt 






Au 


Hg 


Tl 


Pb 


Bi 


— 


— 








— 


Ra 


— 


Th 


— 


U 


— 





which have resulted *'by gradual condensations from a 
primary material." Until such a transformation of one 
element into another has been proven by experiment, 
chemists will still hold to the idea of indivisible elementary 
particles. 

In the periodic arrangement of the elements, their phjrs- 
ical and chemical properties are developed by the position 
of the element in the system. The most electro-positive 



THE THEORIES OF CHEMISTRY. 89 

elements are placed to the left in each group, that charac- 
ter then vanishes as an acid-forming character appears 
until finally the strongly electro-negative elements (the 
non-metals) are found on the right in the group. Valency 
also finds full expression in this system. The table given 
on page 88 is the arrangement of the elements accord- 
ing to the Periodic Law. 

Having brought the work of determining atomic 
weights of the elements up to the present, this subject 
of chemical research will be closed and a return made 
to 1800 in order to follow the second line of chemical 
endeavor which had its inception at that time : 

Theories Formulated to Explain Chemical 
Composition. 

Lavoisier Berzelius Davy-Liehig-Graham 

Radicals Earlier Radicals Definition of Acids 

(CH3)20 Dualistic Theory 

Dumas Berzelius-Liebig-Wohler 

^therin Theory Older Radical Theory 

Dumas-Laurent Laurent Dumas 

Substitution Theory Nucleus Theory Older Type Theory 

Berzelius Gerhardt 

Theory of Copulas Theory of Residues 

Gerhardt-Laurent Kekule Kolhe 

Type Theory Extension of the New Radical Theory 

Type Theory 

Kolhe- Frankland- Couper -Kekule 
Theory of the Saturation Capacity of the Elements or The Theory of 

Valency 

Immediately following the announcement of the 
chemical atomic theory (1802 — ), it became evident to 



90 THE THEORIES OF CHEMISTRY. 

chemists that a more thorough knowledge of the natiire 
of compoimds was imperative. As a result of the investi- 
gations of eminent chemists, who entered this field of 
chemical inqtiiry, theories were formulated which sought 
to explain the composition of these compound bodies. 
These theories were accepted and, as time passed on, were 
either merged into more plausible hj^otheses or were 
dropped as they became inadequate. 

At times, Dalton's atomic theory and its underlying 
principle — the atom — ^was forgotten. The observations 
of Gay-Lussac and others were laid aside. The hypothesis 
of Avogadro was neglected. 

During this period a struggle for supremacy of doctrine 
was carried on by Berzelius and his school on one side 
against the French School on the other. It was a time of 
personal controversies; and yet withal there resulted a 
clearing of the sky at its close. 

The first statement concerning the composition of 
compoimd substances was uttered, before 1802, by 
Lavoisier. He stated, "that the constitution of organic 
substances depended upon the existence of complexes or 
radicals in union with oxygen." Lavoisier was, therefore, 
the originator of the term "radicals." He emphasized 
the fact that oxygen was the important element in both 
inorganic and organic compotmds. Berzelius accepted 
this idea that compounds were composed of radicals and 
oxygen, but added "that sulphur, or the halogens or 
negative groups of elements can be substituted for the 
oxygen which is without the radical." Berzelius was 
influenced by experiments which he had been conducting 
with the electric current upon inorganic compotmds. 

In 1818, Berzeh'us published his electro-chemical 
theory; and it is not strange that he combined the main 



THE THEORIES OF CHEMISTRY. 91 

point of that theory with his ideas concerning "radicals." 
In 1820, he announced his "dualistic theory" which 
sought to explain the composition of compound sub- 
stances. 

The formula (CH3)20 contains the germ of the guiding 
thought of all the theories entertained by Berzelius. 
In these theories the dominant idea is that compoimds 
consist of two distinct parts which are electrically differ- 
ent. Berzelius applied this thought in explaining the 
constitution of inorganic compoimds, for example, oxides 
were composed of the radical (metal or metalloid) and 
oxygen; salts were composed of a basic oxide (electro -{-) 
and an acid radical (electro — ) BaO.SOs, ZnO.C02. He 
further sought to explain the composition of organic 
compounds by the same hypothesis, which were said to 
consist of radicals in imion with oxygen, sulphur, bromine, 
chlorine or groups of negative elements. 

It is remarkable that the idea of radicals, first suggested 
by Lavoisier, is fotmd in all of the theories formtdated 
during the next sixty years. It is very evident in the 
nucleus and substitution theories, and it is even present 
in the type theory. 

It is difficult to trace the development of any one of 
these theories alone. They are so intertwined that it is 
absolutely necessary to speak of them together. They 
are related, and these relations must be emphasized, and 
this cannot be done without frequent repetition. 

In 1820, when the dualistic theory was completed, 
it had been extended to include the organic substances. 
Berzelius did not favor the idea that organic bodies were 
primarily due to the action of a living or vital force. 
He believed that the combination of elements into groups 
or radicals wus the primary cause of their existence. 



92 THE THEORIES OF CHEMISTRY. 

The first manifestations against the dualistic theory 
came from a consideration of "acids" begun by Davy, 
Gay-Lussac and Thenard. Lavoisier had stated that 
oxygen was the "acid principle." 

Davy's discovery of the alkali metals and his experi- 
ments upon chlorine were the initial events in establishing 
the correct nature of acids and their salts. He thought 
that the alkali metals were elements, but after Gay- 
Lussac and Thenard proved the presence of hydrogen 
in ammonia, Davy was in doubt concerning these metals, 
and thought that they, too, might contain hydrogen. 
Gay-Lussac and Thenard proved the alkali metals to be 
elements. 

Berthollet and Lavoisier believed that hydrochloric 
acid contained oxygen. When Davy, Gay-Lussac and 
Thenard began their investigations, hydrochloric acid 
gas was thought to contain chemically combined water. 
These three investigators failed to find oxygen either in 
the acid or in chlorine. Chlorine was thereafter added to 
the list of elements as well as iodine and fluorine. Hydro- 
chloric acid they demonstrated to consist of hydrogen and 
chlorine only. Berzelius was not convinced. He believed 
in the oxygen theory of acids and he v/ould not accept 
these statements as he saw in them a blow to his dualistic 
theory. 

Davy and Dulong were making the attempt to bring 
the acids without oxygen and those containing oxygen 
upon some common ground. 

Davy observed that iodic anhydride did not possess 
acid properties, but acquired them after union with 
water; he, therefore, concluded that hydrogen and not 
oxygen was the ''acidifying principle." This was the 
beginning of the hydrogen theory of acids. 



THE THEORIES OF CHEMISTRY. 93 

These opinions of Davy, concurred in by Dumas, were 
neglected for a time. In 1825, Berzelius had accepted, to 
some extent, the views of Davy and others, for he now 
included fluorine, chlorine and iodine among those ele- 
ments which form salts. He drew a distinction, however, 
between haloid salts (salts which contained no oxygen) and 
amphid salts (salts which contained oxygen). 

In the third decade of the nineteenth century, the 
opinions of Davy and Dumas concerning acids reappeared 
with new proofs. In order to explain the composition of 
acids upon a common basis, a definition was given which 
stated "that hydrogen was always a constituent of acids, 
and with it there was a radical." "The radical did not 
always contain oxygen." This so-caUed ''hydrogen theory 
of acids" or the "hydrogen acid theory," of Davy was 
more clearly explained by Liebig, who used for the 
explanation a series of organic acids and studied the 
saturating capacity of each, and from these studies, he 
suggested that there was a possibility that a molecule of 
acid might combine with more than one molecule of base, 
and that among the organic acids there were some which 
perhaps contained more than one hydrogen atom with 
the power of exchangeability. He designated acids as 
monobasic, dibasic, tribasic, tetrabasic or pentabasic, 
according to the number of exchangeable hydrogen atoms 
contained in one molecule of the acid. Liebig, in this 
connection, profited by the work of Graham on phosphoric 
acid. 

Liebig maintained that "acids are particular compounds 
of hydrogen, in which the hydrogen can be replaced by 
metals." Salts *'are the products of such replacement 
of the hydrogen of acids. Neutral salts were those com- 
pounds in which the hydrogen is replaced by its equiva- 



94 THE THEORIES OF CHEMISTRY. 

lent in metal. Acid salts were those in which there is a 
residuum of hydrogen atoms." 

Salts should not be looked upon as consisting of basic 
oxides and acid oxides (Na20.S03). They were, rather,, 
the products which resulted from the replacement of the 
hydrogen of acids previously in union with an acid radical. 
In this way Liebig demonstrated the similarity between 
the so-called oxygen acids and the haloid acids, and 
between the amphid and haloid salts. He disagreed with 
the dualistic notions of Berzelius. 

Berzelius maintained that the haloid salts were in a 
class by themselves; that the amphid salts consisted of 
two parts — ^an acid oxide and a basic oxide. Barium 
chloride (BaCl2) was a haloid salt, baritmi sulphate 
(BaO.SOa) an amphid salt. Liebig, however, had placed 
all salts in one line. Berzelius objected to Liebig's imitary 
idea concerning salts. This caused an estrangement 
between Berzelius and Liebig. Berzelius always contended 
that Liebig's theory of acids and salts *'led to the con- 
fusion of ideas, and stood in the way of a more perfect 
knowledge." 

During the years following 1810 and especially those 
immediately following 1820, organic chemistry was 
growing in importance. Berzelius held that organic 
compoimds like the inorganic were binary. In the 
former, compound radicals were joined together and these 
played the same r61e in the organic as the elements in 
the inorganic compounds. He thus extended his dualistic 
theory to include organic compoimds. 

Between 1820 and 1830 the first efforts were made to 
explain the composition of organic compounds by means 
of compound radicals, and these subsequently led to the 
establishment of the radical theory. 



THE THEORIES OF CHEMISTRY. 95 

Gay-Lussac had investigated "cyanogen" and showed 
that as a compound radical it played the part of an 
element. He also expressed the opinion that alcohol 
consisted of ethylene and water. It had been observed 
that alcohol could be transformed into ether and ethylene. 
It was assumed, therefore, that ethylene was a constituent 
of both alcohol and ether. This idea of Gay-Lussac was 
further advanced by Dumas and Boullay. 

In 1821, Dumas annoimced that ether consisted of 
C2H4 and H2O. Berzelius was willing to accept this 
because he could compare it readily with NH3.H2O. 
Berzelius named the radical (C2H4) — **^therin." As a 
consequence, Dumas formulated his ''astherin theory," 
the basis of which was "that densities of alcohol and 
ether equaled the sums of the densities of ethylene gas 
and water:'* 



CH2 CI12 
Ether { | = 1 1 = H2O. Two volumes of ethylene plus one of water. 
CH2 CI12 

CH2 
Alcohol II =H20. One volume of ethylene plus one of water. 
CH, 

CH2 
Ethylene chloride 1 1 =HC1. 
CH2 

The radical "aetherin" was thought to be present in 
all ethyl compoimds; and it was compared to the inor- 
ganic radical "ammonia — (NH3)." 

Ammonium hydroxide NH3 = H20 analogous to alcohol. 
Ammonium chloride NH3=HC1 analogous to ethyl chloride. 



96 THE THEORIES OP CHEMISTRY. 

Although this view of the constitution of organic 
compounds harmonized in a certain sense with the idea 
of BerzeHus as set forth in the duaHstic theory, he 
was unwilling to subscribe to the aetherin doctrine as 
finally stated by Dumas. This theory was not generally 
accepted. It lasted but a short time, chiefly because it 
could not be applied to the many new organic compounds 
which were being discovered. Another reason for its 
short life was that, according to it, C2H4 shoidd be a 
very strong base and with water give a strong alkaline 
reaction; whereas alcohol (the body resulting from the 
union of C2H4 with water) was neutral. 

The "setherin theory," however, was the forerunner 
of the radical theory, in that it attempted a comparison 
upon common groimd between organic and inorganic 
compounds. The development of the idea that organic 
compoimds are composed of radicals was brought about 
principally by the researches of Liebig and Wohler. 

They annoimced the discovery of the radical of benzoic 
acid, it had the composition (CoH-^COy and to it they 
gave the name "benzoyl." In the many transformations 
made with the **oil of bitter almonds," they found that 
this radical remained imaltered. This discovery not only 
aided in the development of the radical theory, but it 
exerted a wonderful influence upon the development of 
organic chemistry. 

Berzelius was delighted with the discovery. He sug- 
gested that the radical be named "proin or orthrin" 
because it heralded "the dawn of a new day for organic 
chemistry." 

When "benzoyl" was discovered, there was great 
rejoicing among the friends of Berzelius, and there was 
still greater joy when Wohler and Liebig found that the 



THE THEORIES OF CHEMISTRY. 97 

radical would combine with hydrogen, with chlorine, 
with hydroxyl, with the alcohol residues and that it was 
also possible for the radical to enter into union with itself. 

In Hoffman's ''Faraday Lecture on Liebig," when he 
discusses this discovery of the compoimd radical "ben- 
zoyl" his enthusiasm for these two chemists and for the 
results of their investigation is unboimded. 

One can readily understand how the idea of compound 
radicals, as the fimdamentals of organic chemistry, must 
have taken hold upon the minds of men. Liebig and 
Wohler compared the radical "benzoyl" with potassium 
and other metals in the inorganic compounds. They 
accordingly wrote the oil of bitter almonds CgHbCO.H. 
The following is a list of compounds showing the trans- 
formations of this radical "benzoyl:" 

CeHsCO.H Oil of Bitter Almonds 

CeHsCO.OH Benzoic add 

CeHsCO.Ci Benzoyl chloride 

CgHbCO.NHs Benzamide 

C6H5CO.OC2H5 Benzoic ether 

CeHsCO.CN Benzoyl cyanide 

The immediate outcome of this discovery of the two 
German chemists was to introduce into theoretical chem- 
istry the idea of compound radicals, which in organic 
chemistry act precisely as the metals do in the salts of 
inorganic chemistry, and, to urge chemists to search for 
other similar groups or radicals. 

From such endeavors arose the "radical theory." The 
compounds related to alcohol furnished the best group 
with which to work. At this time, about 1833, Dumas 
believed that "aetherin" was the radical of alcohol and 
ether. Berzelius thought that alcohol and ether had 



98 THE THEORIES OF CHEMISTRY. 

different constitutions. Liebig believed that the "ethyl" 
radical (written by him — C4H10) was common to both 
of these compounds. They all agreed, however, that 
compound radicals existed as distinct constituents of 
these compoimds. The following facts, they held, must 
be true before a group of elements could be considered 
a "radical:" 

(1) It must be either an element or a group of com- 
bined elements, which group remains after one or more 
elements have been removed from the compound. 

(2) It must be possible to replace the radical by a 
simple substance. 

(3) If the radical is in combination with a simple 
substance, it must be possible to replace that substance 
by some other simple substance. 

In addition to these facts, Berzelius claimed that a 
radical was unalterable, but Liebig did not wholly concur 
in this belief. 

The year 1837 fotmd the radical theory at its height. 
Dimias was convinced of the uselessness of his aetherin 
theory. He joined Liebig with a view of investigating 
organic substances. They both regarded ^'organic 
chemistry as the chemistry 0} compound radicals.''^ 

In 1839, Bunsen's investigations of cacodyl compounds 
considerably aided the radical theory. 



The preceding paragraphs give the state of affairs 
existing when Dumas began investigating radicals. 
Gay-Lussac, Faraday, Liebig and Wohler had noted the 
action of chlorine upon certain organic compounds and 



THE THEORIES OP CHEMISTRY. 99 

had observed that hydrogen escaped in amount equiva- 
lent to the chlorine which entered the compound. 

In 1834, Dumas, after investigating the action of 
chlorine upon turpentine and the production of chloral 
from alcohol, declared: 

''When a compound containing hydrogen is exposed 
to the action of chlorine, bromine, or iodine, it takes up 
an equal voltime of chlorine, or bromine, etc., for each 
atom of hydrogen that it loses. 

*'If the compound contains water, it loses the hydrogen 
of this without replacement." 

Dumas, in 1835, extended this statement to one which 
is the basis of his "substitution theory" — "that in 
chemical reactions, generally, an exchange of equiva- 
lents of one element for equivalents of others takes 
place." 

Latirent proceeded further, he stated, ''that the struc- 
ture and chemical character of organic compounds are 
not materially altered by the entrance of chlorine and the 
separation of hydrogen." This observation together 
with the view that chlorine takes the place of the 
substituted hydrogen constitute the core of the substitu- 
tion theory. The theory belongs more to Laurent than 
to Dumas, for the latter, when attacked on this point 
by Berzelius, denied having any share in that portion 
of the theory which stated that the chlorine in taking 
the place of the substituted hydrogen caused no material 
change in the character of the compoimds thus acted 
upon. 

The discovery of trichloracetic acid (C CI3. COOH) 
by Dimias gave considerable aid to the establishment 
of this new doctrine. 

Thus the fact that chlorine could be introduced into 



100 THE THEORIES OP CHEMISTRY. 

acetic acid for hydrogen, and an acid still remain which 
would form salts with metals, led Dumas to agree with 
Laurent, his pupil, that the only change which the acid 
had sustained was that the positive hydrogen atoms had 
been replaced by negative chlorine atoms. Berzelius 
was unable to reconcile this thought with the dualistic 
notions based as they were upon his electro-chemical 
theory. The idea of replacing electro-positive elements 
with electro-negative elements was untenable. He, 
also, considered that a compound in which oxygen was a 
fourth element was at once an oxide and a sesquichloride ; 
therefore, trichloracetic acid was, in his opinion, a copu- 
lated compound of sesquichloride of carbon with sesqui- 
oxide of carbon. 

Dumas said, however, "that acetic acid was at low 
temperatures a leafy, crystalline mass, melting at 16.7 
and yielding at the same time a penetrating acid-like 
liquid." The trichloracetic acid made from this acetic 
acid by the action of chlorine upon it in the sunlight, 
was also a body that crystallized and showed properties 
similar to those manifested by the parent, acetic acid. 
The only way to account for the existence of this body 
was on the assumption that the negative chlorine had 
taken the place of the positive hydrogen. 

Berzelius asserted that trichloracetic acid had not been 
produced by substituting negative chlorine for positive 
hydrogen in the parent acid, but that it had been formed 
by chlorine atoms attaching themselves without the 
radical. 

Gerhardt and Laurent, at this time, contributed much 
of the experimental work which in the end brought about 
the acceptance of the ideas embodied in substitution and 
caused the overthrow of dualism. Berzelius was exhaust- 



THE THEORIES OF CHEMISTRY. 101 

ing himself with fruitless discussions, while in France 
these two men (Gerhardt and Laurent) were making 
discoveries and accumulating evidence against the dual- 
istic notions which Berzelius was so zealously guarding. 

Malaguti and Regnault had prepared dichlorformic 
ether and other complex bodies, which could not be 
explained by the idea of dualism, but which could be 
made clear on the basis of substitution. 

In 1842, Melsens, by converting trichloracetic acid 
into acetic acid, gave decisive evidence as to the truth 
of the principle of the substitution theory. During the 
years (1832-1842), there was a division of opinion among 
chemists as to the relative merits of the existing theories. 
Some inclined to the French School ; others to the ideas of 
Berzelius, Liebig or Gerhardt; and others, taking no active 
part, waited for the heavens to clear. It was then that 
Wohler, for the first time, permitted himself to express 
an opinion in regard to a debated question. A letter 
signed S. C. Windier appeared in Liebig's "Annalen:" 
Wohler wrote it as a private commimication to his friend 
Liebig. It was dated from Paris. In it the writer an- 
noimced: **I have succeeded in replacing in manganese 
acetate not only the hydrogen by chlorine, but also the 
metal, and finally the carbon and oxygen too ; the product, 
although consisting of chlorine only, exhibited all the 
characteristic properties of the original salt ... by 
passing chlorine in the sunlight into a solution of copper 
acetate, I obtained, as was to be expected, the copper 
salt of Dumas' chloracetic acid. On heating this in a 
current of dry chlorine, oxygen was liberated, a yellowish 
mass being formed consisting of chloro-copper chlorace- 
tate. On continuing the action of chlorine the metal 
was also replaced and chlorine chloracetate was formed, 



102 THE THEORIES OF CHEMISTRY. 

crystallizing in small, brilliant, golden-yellow prisms. 
This was, however, not the final product, inasmuch as by 
treating it in an aqueous solution with chlorine for two 
weeks longer, carbolic acid was evolved, and in cooling 
the liquid to 2° it deposited crystals, having all the prop- 
erties of chlorine hydrate." 

To which may be added a few lines of a letter from 
Wohler to Berzelius in 1840: 

"The chemical bother and talk, the eternal song of 
the substitutions makes me quite sick, and how many of 
the statements are only guess-work, only assertions, 
and yet put down as facts." 

Throughout all of the discussions upon substitution, 
Berzelius insisted that electro-negative elements could 
not displace electro-positive ones. According to the 
recent assertions of Julius Thomsen, there exists in the 
chlorine molecule, electro-negative chlorine in imion 
with electro-positive chlorine. The same is true of 
the hydrogen molecule. When, therefore, chlorine 
enters a compound, such as benzene, and substitutes 
hydrogen, it displaces electro-positive hydrogen and 
electro-positive chlorine enters to combine with the 
electro-negative hydrogen that remains. If this is true, 
Berzelius was correct. 



Liebig abandoned the dualistic ideas of Berzelius slowly. 
He and Wohler had contributed much in laying the 
foundation of the radical theory; and an organic chem- 
istry which was a chemistry of compound radicals was 
wholly acceptable to them. The chemists of the French 
School were, however, discovering many new substances 
which could only be regarded as substitution products. 



THE THEORIES OF CHEMISTRY. 103 

As time progressed, Liebig, Wohler and others reluctantly 
renoimced dualism and subscribed to the main points of 
the substitution theory. 

Immediately following the general acceptance of 
Dumas' substitution theory, Laurent advanced a new 
theory — the "nucleus theory" — ^first published in 1836. 

Laurent claimed that "All chemical bodies have a 
definite groimd form. They consist of a central nucleus, 
in which the individual elements can be replaced by other 
elements, and grouped around this central nucleus are 
other nuclei." 

According to Laurent, compoiinds had form. He was 
the first to suggest "space chemistry." His explanations 
are not clear, but they are the first indications of 
* ' geometrical isomerism . ' ' 

According to Wohler, Laurent's nucleus theory was 
based entirely upon substitution. 

This theory never met with very general approval. 
It was based upon the radical theory and included the 
idea of substitution, but it vaguely suggested a new 
thought^that of space chemistry. 

Laurent proved that radicals could be altered. His 
efforts to classify organic compoimds on imiform princi- 
ples were helpful. His views would have fallen into 
oblivion if it had not been for Odling and Gmelin. 
Gmelin, in Germany, thought the teachings of the 
French School merited consideration; he was so deeply 
impressed with Laurent's nucleus theory that he included 
it in his text-book. Odling was determined that the 
ideas of Laurent should reach the chemical students of 
England; he translated Laurent's '* Chemical Method," 
which was published by the Cavendish Society, as was 
also a translation, by Henry Watts, of Gmelin's text- 
book, "Hand-Book of Chemistry." 



104 the theories of chemistry. 

Extracts From the Hand-Book of Chemistry by 
Leopold Gmelin. 

"The atoms of organic compounds are either nuclei 
or compounds of nuclei with various substances attached 
to them externally. — Laurent. [It is only fair to say 
that Latirent frequently designated nuclei by the term 
radicals. This application has a tendency to confound 
them with the radicals of the radical theory, from which 
they differ in reality very materially. The nuclei contain 
an even number of atoms, the radicals an uneven number of 
atoms. — Gmelin.] 

" The number of carbon atoms in the nucleus bears 
a relation to that of the other atoms. If the nuclei 
contain hydrogen, in addition to the carbon, they are 
primary nuclei. But if one or more of the hydrogen 
atoms of the nucleus are replaced by atoms of other 
elements or of certain compounds, organic or inorganic, 
which take the same place in the nucleus that the hydrogen 
atoms originally occupied, the compound atoms thus 
formed are called derivatives or secondary nuclei. Hence, 
in the theory of nuclei, less importance is attached to the 
nature of the elements than in the radical theory; more 
importance being attached to their configuration. The 
elements, which usually replace the hydrogen atoms in 
the nucleus are chlorine, bromine, iodine, oxygen, nitrogen 
and the metals. When the hydrogen of a nucleus is 
replaced by a compound, each atom of hydrogen is 
replaced by one atom of the compound. The compoimds 
capable of replacing hydrogen in the nucleus are NO^, 
NH^ C^N, etc. . . . When, by the action of various 
substances, one nucleus is converted into another without 
loss of carbon, the new compoimd cannot be represented 



THE THEORIES OF CHEMISTRY. 105 

by a formiila, in whidi the nucleus is supposed to contain 
a number of carbon atoms different from the former. 
When, on the contrary, a portion of the carbon separates 
from the nucleus in the form of carbonic acid, the old 
nucleus must be replaced by another containing a smaller 
number of carbon atoms and, therefore, standing lower 
in the organic scale. When decomposition takes place 
in such a manner that the hydrogen, withdrawn from the 
nucleus, is replaced by an equal number of atoms of 
chlorine, etc., the nucleus remains the same, but is trans- 
formed more or less into a secondary nucleus. When, 
however, hydrogen, or chlorine, etc., is withdrawn with- 
out substitution, the residue, if still an organic compound, 
must belong to the series of another nucleus. The re- 
placement of a certain niunber of atoms of hydrogen in 
the nucleus by chlorine, bromine or iodine does not alter 
the elements of the compound as much as the replacement 
of an equal ntimber of atoms of hydrogen, by oxygen, 
etc. Moreover, all nuclei are neutral, even when they 
contain oxygen, chlorine, bromine, NO^, etc." — ^Laurent. 



It was in the 30's when Laurent's theory of nuclei was 
introduced, and he declared that it was possible to intro- 
duce NO2 for one atom of hydrogen in a compoimd 
because the space occupied by the hydrogen atom with 
its heat sphere is sufficient space for the other substance 
with its heat sphere, provided the heat sphere of the 
atoms of the new substance entering take up less space 
than the hydrogen or an equal space. Fiurther, chlorine, 
bromine, iodine or sulphiir might combine with the 
nucleus to the number of from two to six atoms. These 



106 THE THEORIES OF CHEMISTRY. 

elements may be removed by potash without the substi- 
tution of another substance. 



"These additions of chlorine to the nucleus may be 
removed by the potash without substitution of another 
substance. If ethylchloride is treated with caustic potash 
the hydrochloric acid is drawn out to form potassium 
chloride and water, and the original nucleus C^H^ is 
restored. ' ' — Laiirent. 



It is now known that C2H4 treated with hydrogen 
bromide or chloride yields monochlorethane or mono- 
bromethane. Laurent and the chemists of his day did not 
know these unsaturated bodies of the first, second and 
third degrees. 



"Laurent's Classification of Organic Compounds. 

"Organic compounds may be arranged in series. The 
basis of each of these series is a primary nucleus, together 
with its secondary nucleus. The compounds of the same 
series contain the same number of atoms of carbon." 
For example, the ethylene series, — ethylene — C^H^ [C2H4]. 
When water is added externally to ethylene, alcohol 
results— C4H^ H^O^ [C2H5OH]. 

"Each series contains compoimds belonging to differ- 
ent types. The nucleus type includes all nuclei, primary 
or secondary; the alcohol type, all nuclei to which water 



THE THEORIES OF CHEMISTRY. 107 

(H^O^) has been added; the monobasic acid type, all 
nuclei which have taken up 4 O in addition and so on. 

"A primary nucleus may be altered by abstraction 
of 2H; the remainder is the Characteristic (Car.), which, 
in the case of methylene, C^ H^, is merely C^, while in 
other nuclei it is C^H- the remainder of the hydrogen. 
The 2H are the Constant; so that every primary nucleus = 
Car. -{-Const. = Car., H^. Hence, the compoimds of any 
series may be arranged and designated as follows: — 
Nuclei. . . Halydes. . . Hydrides. . . Anhy- 
drides. . . Holoformes, Amides, etc." — Laurent. 



''I have considered it my duty to explain the more 
important doctrine of Laurent's theory," says Gmelin, 
"more especially as the theory has not in Germany, at 
least, the attention it merits. Whoever will submit it 
to the test of examination will admit that the nucleus 
theory of Laurent is the one that gives the most simple, 
the most comprehensive view of the many thousands of 
organic compoimds and unites them with the most 
natural families or series. With my own idea this theory 
is in peculiarly close accord." 



Laurent had endeavored to represent substances by 
means of figures: "Imagine a right sixteen-sided prism, 
and in each angle one atom of carbon, making in all 
thirty-two atoms of carbon; in the middle, between each 
two angles, one atom of hydrogen, making altogether 
thirty-two atoms of hydrogen; and lastly, on each base 



108 THE THEORIES OF CHEMISTRY. 

of the prism, one atom of OH, forming pyramids. Thus 
the compound equals C^2H22.2 OH. In the same manner 
as crystals may be cleft mechanically, and the primary 
nucleus separated from the secondary envelope, . . . 
so likewise, may a chemical separation be made. After 
the removal of the two pyramids OH, there remains the 
primitive form (the germ or nucleus). Chlorine or oxygen 
acting on this primitive form withdraws the hydrogen; 
the prism would then fall to pieces, if the hydrogen atoms 
were not replaced by atoms of chlorine or oxygen. The 
HCl or OH may either pass off or foiTQ pyramids, on the 
prism, which, however, may be removed by chemical 
division." — ^Laiirent. "Further than this, Laurent . . . 
made no attempt to assign definite figures to particular 
compounds. Gerhardt . . . considerably extended 
them. He suggested that an effort be made to apply 
the nucleus doctrine to inorganic compounds. If it 
should answer in the inorganic field, it will be 
strengthened." — Gmelin. 



Gmelin, using Laurent's nucleus theory as a basis, 
discusses the relative position of the elementary atoms 
in inorganic compounds. He greatly extended Laurent's 
ideas concerning the arrangement of atoms in space. 
Gmelin is uttering the first important statements in 
spacial chemistry thirty years prior to the first annoimce- 
ments made by van't Hoff and Le Bel (1874). 

Concerning inorganic compounds, Gmelin writes, "The 
crude chemical formula of sulphate of potash is KSO* 
(K2SO4), the rational formula of Berzelius is KO, SO^ 
(K2O.SO3), if the compound be regarded as sulphate of 



THE THEORIES OF CHEMISTRY. 109 

potash; K, SO^ (K2. SO4) if it be regarded as sulphanide 
of potassiiim; and KS, O^ (K2S. O4) if it be regarded as 
oxidized sulphide of potassiiim. Now whichever of these 
three rational formula be adopted the mode of writing 
the formula gives no satisfactory idea of the manner in 
which the atoms are actually combined. The three 
substances in the formula might be arranged in a straight 
line, KO, SOO (K2O.SOOO), but in nature they are 
doubtless united in a body of three dimensions; for their 
mutual affinity induces the greatest possible approxima- 
tion of the heterogeneous atoms. Nearly all chemists 
adopt the atomic theory; they determine the relative 
weights of the atoms, and their relative distances one 
from the other, or the relative spaces occupied by each 
atom of the compoimd substance, including the surroimd- 
ing calorific envelope; hypotheses are also made respecting 
the form of the atoms, etc. . . . Why then should 
we not throw out suggestions in regard to relative positions 
of these atoms." 

Gmehn continues, "It may be assumed that the heter- 
ogeneous atoms of a compound will approach as near to 
each other as their afhnity requires, and the elasticity of 
the calorific envelope allows, and that they will take up 
that particular position with regard to each other, which 
allows of the greatest and most varied approximation of 
the heterogeneous atoms. Two atoms, such as hydrogen 
and oxygen, H and O, can only be disposed in a line, 
horizontally or vertically. The same is true in regard 
to two atoms of one substance and one atom of another 
substance MnO^, here the manganese lies in the middle, 
one atom of O to the right and one atom of O to the left, 
0-Mn-O. One atom of one substance and three of 
another, such as SO^ would probably form a plain triangle 



110 THE THEORIES OF CHEMISTRY. 

with the sulphur atom in the middle. With one atom and 
four atoms, as in NO^, a tetrahedron can probably be 
formed. Nitrogen in middle and oxygen at the four 
comers. With one atom and five atoms as PO^ (P2O5), 
we wotild have phosphorus in the middle, one atom of 
oxygen above and one atom below and three atoms of 
oxygen disposed horizontally round the phosphorus atom. 

"When this and similar compounds of the first order 
unite together, for example to form salts, we may imagine 
that in many cases the relative position of the atoms no 
longer remains the same, but is so far altered that the 
tendency of the heterogeneous atoms to approximate 
as closely as possible is satisfied to the utmost. Thus, 
in the combinations of KO (K2O) and SO^, a compound 
atom is probably formed, in which the sulphiu: is placed 
upon the potassitmi and the three oxygen atoms are 
horizontally arotmd the points of contact. Similarly 
with KO, SO^ (K2O. SO3), excepting that in this instance 
the four oxygen atoms are placed in a square aroimd the 
point of imion of KS, so that a double four-sided pyramid 
is formed. . . . Use balls of wax," says Gmelin, 
** variously colored to represent different atoms and then 
arrange them as just suggested." Is not this space 
chemistry? To-day, models are used in order to explain 
the peciiliar isomerides, and the bodies which occupy the 
solid angles are differently colored from those which 
occupy central positions. 

"The tendency of heterogeneous atoms to approximate 
as closely as possible, causes them to assume the most 
simple arrangement that their number will admit. That 
the crystals of sulphate of potash assimie in spite of this 
arrangement of the double four-sided pyramid, a form 
belonging to the right prismatic, instead of the square 



THE THEORIES OP CHEMISTRY. Ill 

prismatic system, arises perhaps from the sulphur having 
a much larger atomic number than the potassium, and 
consequently a much smaller specific or atomic volume, 
in consequence of which the vertices of the two pyramids 
are dissimilar. 

"This supposition respecting the aggregation of atoms 
in sulphate of potash, may perhaps terminate the con- 
troversy as to whether that compoimd is KO,SO^ (K2O.SO3) 
or KS, O'* (K2S. O4). According to the above hypothesis 
it is neither of the three, but rather KO^S (K2O4S). The 
atoms, indeed, are imited in such manner that we cannot 
assert which oxygen atoms belong to the sulphur and 
which to the potassium. At first view, it might appear 
that the third formula KS, O^ (K2S. O4) is admissible, but 
in sulphide of potassium, the potassium and sulphur are 
probably more closely imited than in the sulphate of 
potash, in which the foiu: surrounding oxygen-atoms 
interpose themselves to a certain extent between the 
potassium and the sulphur. For the rest, until the above 
hypothesis shall have been put to the test of experience, 
which may be best done by comparing the crystalline 
forms of salts with the assumed form and arrangement 
of their atoms — ^preference must be given to the first of 
the three preceding formulae, namely KO, SO^ (K2O. 
SO3), inasmuch as it has always been adopted hitherto, 
and in the rest, the objections outweigh the advantages. 
Oil of vitriol would be similarly constituted, the potassium 
being merely replaced by hydrogen. 

**This theory may perhaps lead to an explanation of 
the three isomeric states of phosphoric acid. 

"If we now apply to organic compounds the principles 
above illustrated, by examples taken from inorganic 
chemistry, and assume with Laurent, that these compounds 



112 THE THEORIES OF CHEMISTRY. 

may be divided into organic nuclei and compounds of 
these nuclei with substances externally attached to them, 
we shall find that, in the nuclei, the carbon-atoms must 
be imited with the other elements composing the nucleus, 
in such manner as to allow the heterogeneous atoms to 
approach the nuclei in as many ways as possible, whereby 
a determinate figure must be produced. 

"The nucleus, ethylene C^H^ (C2H4) may serve as an 
example. It probably has the form of a cube, four angles 
of which consist of carbon atoms, and the four others 
diametrically opposed to them of hydrogen atoms. . . . 
To the ethylene series belong among other compotmds 
aldehyde, alcohol and acetic acid, the crude formulae 
of which are C^H^O^, C^H^O^ and aWO' (CH3CHO, 
C2H5OH, CH3COOH). These three compounds may, 
with some degree of probability, be supposed to contain 
the secondary nucleus C^H^O, one angle of the cube 
consisting of oxygen. This cube-summit formed of an 
atom of oxygen may be called an 0-pole; and the summit 
diagonally opposed to it and formed of carbon, a C-pole. 
According to this supposition, the rational formula of 
alcohol would be aH^O, H^O, ... The 3 H, in 
consequence of their peculiarly strong affinity for the O, 
are disposed on those three faces of the cube, one angle 
of which is formed by the 0-pole while the external 
0-atom places itself upon the carbon-pole. 

"The rational formula of aldehyde, determined in 
similar manner, is C^H^O, HO; the H-atom is attached 
to the 0-atom of the nucleus, and the 0-atom to the 
C-pole. . . . 

"Acetic acid, would in the same manner be regarded 
as OH^O, H03 or more precisely, as O^, C^H^O, H, the 
3 being disposed on those three faces of the cube 



THE THEORIES OF CHEMISTRY. 113 

which consists only of C and H atoms, and 1 H on the 
0-atom of the nucleus. 

''Alcohol, by the action of 2 O, is converted into alde- 
hyde and 2 HO; and this aldehyde, by the further addition 
of 2 O, is converted into acetic acid. The first 2 At. O, 
together with 2 H out of the three external H-atoms, 
form 2 HO; the third of these external H-atoms is trans- 
ferred from the cube-face to the 0-atom of the nucleus, 
and aldehyde is thus formed. If 2 more O are added, 
they dispose themselves on two of the cube-faces con- 
sisting only of C and H, while the 0-atom attached to 
the C-pole is transferred to the third cube-face, and thus 
acetic acid is formed." 



Gmelin thought that when acetic acid was neutralized 
with potash, the resulting compoimd would be " 0^ C^H^O, 
K;" the external atom of hydrogen having been replaced 
by potassiimi, with the formation of water. In this he 
differs from Laurent, who believed that all of the hydrogen 
atoms of acetic acid were within the nucleus, "C^H^O'* 
and that potassium, therefore, entered the nucleus — 
aHsK, 04." 

In explaining the substitution of the hydrogen of acetic 
acid by chlorine, Gmelin says, ''in this case, it is obvious 
that the H-atom which is replaceable by a metal remains 
unaltered. 

"This difference of comportment between the one H- 
atom and the other three is explained by their different 
arrangement." The hydrogen atom which may be 
replaced by metal is different because it touches only 
the oxygen atom of the nucleus. 

** Moreover, in consequence of the very close proximity 



114 THE THEORIES OF CHEMISTRY. 

which exists between the simple atoms of an organic 
compoimd, it is probable that the result is determined 
not only by the affinity of the atoms lying immediately 
together, but also, though in a less degree, by that of the 
more distant atoms. . . . Finally, to consider one 
of the most complicated cases, the formation of a compoimd 
ether: Alcohol and Acetic acid form 2 At. water and 1 At. 
acetic ether. . . . Here we must suppose that the 
carbon-pole of the alcohol first approaches the oxygen- 
pole of the acetic acid, so that the former may give up 
the external 0-atom there situated to the external H-atom 
on the 0-pole of the acetic acid; in the next place, the 
alcohol and acetic acid — a cube-face of the one being 
turned towards a cube-face of the other — ^must turn 
round in such a manner that the C-pole of the alcohol 
and the 0-pole of the acetic acid may move away from 
one another, and the 0-pole of the alcohol and the C-pole 
of the acetic acid approach one another. In this move- 
ment, one of the three external H-atoms surrounding 
the 0-pole of the alcohol, comes in contact with one of 
the external 0-atoms surrounding the C-pole of the 
acetic acid; these atoms unite in the form of a second 
atom of water, and are eliminated; and the cube-face 
of the alcohol thereby exposed attaches itself to the 
simultaneously exposed cube-face of the acetic acid; and 
in this manner 1 At. acetic ether is produced. Hence 
this compound, being formed by the juxtaposition of 
two cubes, has the form of a square prism. On two of 
the opposite lateral edges of this prism, there are situated 
in succession, supposing the acetic acid to be at the top, 
CHCH: on the third: OCHC: on the fourth: HCOC. 
On this last edge, however, there are likewise two external 
O-atoms belonging to the acetic acid, at the top, and two 



THE THEORIES OF CHEMISTRY. 115 

external H-atoms, belonging to the alcohol, at the bottom, 
hence this last edge exhibits this arrangement : 

O H 

H CO C 

O H 

Of the compounds of the "methylene series," Gmelin 
writes, "In this case, we may suppose that methylene 
has the shape of a square table, the four comers of which 
are formed of a C-atom and an H-atom alternately. 
In marsh-gas, two more atoms of H are attached to the 
nucleus, making C^H^, H^ (CH4) ; of these 2 H, one is 
attached to the upper, the other to the lower surface of 
the square table; and if the figure be so placed that one 
of the C-atoms shall be alone and the other below, the 
4 H will be arranged horizontally aroimd the point of 
contact of the 2 C so that an octahedron will be formed. 
"These examples taken from the ethylene- and the 
methylene-series may suffice, for the present, to give a 
general idea of the view which I entertain of the constitu- 
tion of organic compounds. . . . Even if the data 
of this investigation are defective or erroneous, I am 
yet convinced, that aU theories on the constitution of 
organic compounds, and all controversies as to this or 
that mode of writing rational formula, if not supported 
by a plausible arrangement of the compotmd atom, will 
aid us but little in the acquisition of correct ideas." 



The writings of Gmelin are full of interest. The 
nucleus theory is forgotten to-day, but one thought 
contained in Laurent's writings, "that atoms might 
arrange themselves according to a form," has been re- 



116 THE THEORIES OF CHEMISTRY. 

vived in recent times. This idea introduced by Laurent 
was made clearer and greatly extended by Gmelin. He 
explained the constitution of inorganic as well as organic 
compounds by means of ''geometrical figures." The 
moment that the figure of siilphur trioxide is drawn in 
accordance with the recommendation of GmeHn, the 
moment a triangle is constructed and on its solid angles 
are placed balls of colored wax, that moment there is 
being performed precisely what van't Hoff, Pasteur, 
Le Bel and Baeyer did at a much later time. 

The life of the nucleus theory was short. "Substitu- 
tion" was still a subject of discussion. Following close 
upon the nucleus theory, Dumas proposed a theory, which 
he called the ** older type theory." Dtimas was led to 
formulate this theory after his study of trichloracetic 
acid. He concluded that "there are in organic chem- 
istry certain types which remain imchanged, even when 
their hydrogen is replaced by an equal volume of 
chlorine, bromine, or iodine." The term "chemical 
type" did not satisfy Dumas, and he introduced the 
term "mechanical type." The "mechanical type" in- 
cluded all compotmds derived from one another by sub- 
stitution, but differing in properties. 

This theory survived a short time only. Berzelius was 
still opposed to those principles which were contrary to 
his cherished beliefs. Dumas declared Berzelius' "dual- 
istic theory wrong." "Every chemical compound," 
said Dumas, "forms a complete whole and cannot, there- 
fore, consist of two parts. Its chemical character is 
dependent primarily upon the arrangement and niunber 
of the atoms, and in a lesser degree upon their chemical 
nature." 

Berzelius would not accept these utterances. Liebig, 



THE THEORIES OF CHEMISTRY. 117 

who did admit the fact of substitution, would not consent, 
however, to the most extreme assertions of Dumas. 
For five years following 1838, Berzelius tried to combat 
these ideas of substitution and of types; standing, at 
times, upon ground both imsafe and untenable. He woiild 
not agree that trichloracetic and acetic acids had the same 
constitutions. After Melsens' work in 1842, Berzelius 
was compelled to admit that he was in error. He made a 
brave effort to save his cherished ideas in a "theory of 
copulas." Berzelius was, however, compelled to admit 
that chlorine could substitute hydrogen in the copula. 
This theory was much derided and Berzelius foimd that 
his adherents were imwilling to follow him. He then 
suggested that there was far too much speculation and 
not enough experimentation on the part of chemists. 

Laurent had carefully studied all of the existing theories 
before he formulated his theory of nuclei, and he was as 
dissatisfied with that theory as were his contemporaries. 
It will perhaps be of interest at this point to learn how 
Laturent viewed some of the theories which have thus far 
been presented. In a most interesting voltune, entitled 
"Chemical Method," these observations are found: 

"According to some chemists, compotmd bodies are 
formed of two groups of atoms maintained in connection 
with one another by an electrical force, one of the groups 
being positive, and the other negative. 

"According to other chemists, the atoms form but one 
single group, they are arranged therein in a symmetrical 
manner and have a certain relation to the crystalline form 
of the body. I will not stop to inquire whether the atom 
of sulphur is positive towards that of oxygen and negative 
towards that of potassium, whether two atoms when 
brought into union by their opposite electricities can 



118 THE THEORIES OF CHEMISTRY. 

remain indefinitely attached to one another by means of 
the same force, whether atoms have, like the mineral 
tourmaline, two poles — the one positive and the other 
negative — or whether or not they are surrounded by an 
electric atmosphere. One primary question controls 
the whole inquiry. In order to determine the arrange- 
ment of the atoms which constitute a compoimd, it is 
evident that we must first be fully acquainted with the 
nimiber of those atoms and consequently with their 

weight There are greater probabilities in 

favor of the atoms of Gerhardt than in favor of those 
of Berzelius. Consequently, it is probable that nitric 
acid does not contain water, and that dualism is leading 
us into error concerning the arrangement of atoms. 

"The chemists who uphold dualism are far from being 
agreed among themselves. All of them in maintaining 
their opinions rely upon the phenomena of chemical 
reaction, and for a long time the uncertainty of this 
method has been pointed out. It has been shown repeat- 
edly that the atoms put into movement dining a reaction 
take, at that time, a new arrangement, and that it is 
impossible to deduce the old arrangement from the new 
one. It is as if in the middle of a game of chess, after 
the disarrangement of all of the pieces, one of the players 
should wish, from the inspection of the new place occupied 
by each piece, to determine that which it originally 
occupied. 

** It is reiterated everywhere at every moment that salts 
are obtained by combining acids with bases, that we can 
decompose them by separating the acid from the base, 
and that consequently, they contain two groups, the one 
negative and the other positive; the one acid and the other 
alkali. 



THE THEORIES OF CHEMISTRY. 119 

^'Now, let me remark that there is scarcely one salt in 
a thousand that has been obtained by the direct combina- 
tion of its acid with its oxide, and still fewer that can be 
decomposed into an acid and an oxide, for even in making 
potassiiim sulphate, it is always hydrogen sulphate and 
hydrate of potassiimi that are employed. I deny the 
possibility of preparing this salt by means of the an- 
hydrous acid and oxide. I affirm that this often quoted 
illustration has never been submitted to experiment and 
that the majority of salts cannot be made in this way with 
the anhydrous oxides and acids. And, besides, what is 
proved by this possibiHty of preparing certain salts by 
combining directly the acids and bases? If the manner 
in which the salts was formed and decomposed allowed 
us to determine the arrangement of its atom.s we might 
maintain, first, that potassium sulphate is a combination 
of potassium sulphite with oxygen; second, that potas- 
sium sulphate is a combination of potassium sulphide 
with oxygen, since it can be prepared from these two 
bodies, and, moreover, when heated with charcoal it is 
transferred into sulphide; and third, that it contains 
K2SO4, etc. 

**0f all these hypotheses, there is not a single one 
incapable of having reactions adduced in its support. 
Thus, if we choose to adopt the theory of Longchamp, 
according to which the oxygen salts contain the metals 
in the state of binoxide, we could prove by uniting sul- 
phurous acid with binoxide of potassium that potassium 
sulphate contains K2O2.SO2, and we could explain its 
decomposition with chloride of bariiim by the aid of the 
following equation : 

Ka02 . SO2 + BaCl2 = Ba02 . SO2 + KjCIa 



120 THE THEORIES OF CHEMISTRY. 

If we had to explain the formation of potassium sulphate 
from potassium sulphide and oxygen, we could say that 
this last body acted upon the siilphur and burned it as 
if it had been uncombined and at the same time acted 
upon the potassiiim and converted it into a binoxide. 

"We could prove by synthesis that hydrated nitric 
acid is a combination of peroxide of nitrogen with oxide 
of hydrogen. We could prove the same thing analytically 
by decomposing the acid by heat, when it would be resolved 
into peroxide of nitrogen, water and oxygen. In this 
case we should remark that the occurrence of water was 
due to the binoxide of hydrogen having undergone decom- 
position by reason of the heat employed. 

*'An objection might be raised to Longchamp's view 
inasmuch as many of the acids admitted to be present 
in the salts do not exist in the free state, but precisely 
the same objection coiild be advanced against his adver- 
saries. 

"The partisans of dualism do not, however, yield to 
these reasons, and contrive the following means to com- 
bat them : 

"To study the constitution of a binary compound 
we cannot,' say they, 'react upon it with any substance 
that first presents itself, but we must employ for the 
purpose a single body; and in order to study the consti- 
tution of a salt we must employ a base and an acid. Then, 
as bases replace bases and acids replace acids, it is clear 
that salts consist of bases and acids.' 

" We must employ a base and an acid? And why must 
we do so? This they do not tell us, and for my part, I 
choose to employ simple bodies in my study of the con- 
stitution of salts. I choose to put iron in contact with 
sulphate of copper, and as the first metal replaces the 



THE THEORIES OF CHEMISTRY. 121 

second, I embrace the system of Davy, and I conclude 
with him that sulphate of copper is constituted thus: 
Cu . SO4. 

"I now choose to take binoxide of barium and to react 
with it upon the same salt, and as by this means binoxide 
of copper is formed, I maintain with Longchamp that 
sulphate of copper contains sulphurous acid and binoxide 
of the metal : Cu02 . SO2. 

"I now choose to carry out your method. I pour sul- 
phuric acid into potassiimi hyposulphite, and as reaction 
gives rise to sulphur and siilphurous acid I conclude that 
the salt is formed of three groups, S . K2O . SO2, and 
that the trinitary system ought to replace the duahstic 
system. 

"I previously remarked that there is scarcely one case 
in a thousand where anhydrous oxides and acids can be 
employed for the preparation of salts or obtained by their 
decomposition. To overthrow this objection, however, 
seA^eral experiments have recently been made, and the 
old inference arrived at. That is to say, chemists have 
succeeded in decomposing carbonate by anhydrous sul- 
phurous and sulphuric acids, and as the carbolic acid is 
displaced from its combination without the reciurence 01 
water, they have concluded that it existed ready formed 
in this carbonate. 

"It is astonishing how at this day we should be com- 
pelled to attract attention to the most ordinary phenomena 
of chemistry, that we should be obliged to point out that 
one metalloid can displace another analogous metalloid, 
that one metal can drive out another metal, that an oxide 
can drive out an oxide, a peroxide expel and originate an- 
other peroxide, a hydrated acid give origin to another 
hydrated acid, that a salt produces another salt, etc., — ^in 



122 THE THEORIES OF CHEMISTRY. 

one word, that a body that fulfills a certain function always 
does by its action upon a salt tend to displace or give origin 
to another body fulfilling the same function, and if this 
displacement does not invariably take place, it is easy 
to say that the exceptional cases arise solely from the 
instability of the body intended to be thus displaced. 
For instance, at a certain temperature sulphuric acid by 
its action upon nitrate of potash gives rise to hydrated 
acid, but at a higher temperature this last acid cannot 
longer exist and consequently other products are formed. 

*'If, from such simple combinations as the sulphates 
and the carbonates, we pass on to the phosphates and the 
borates and the silicates, we shall still recognize the 
endeavor to construct binary groups, each of which is 
itself composed of other binary groups, but the number of 
atoms become greater and the nimiber of hypotheses 
concerning their arrangement increases in the same 
proportion. 

*'We have a silicate which contains SiisOesMggiAUHso, 
and we discuss seriously whether the atoms have this 
arrangement 

( (11 Si02+21 MgO)+2 (Si02+Al203) + 15 Aq) 

or this 

(7 (Si02+3 MgO)+2 (3 SiOz . Al203) + 15 Aq; 

or this 

(7 (Si02+3 MgO+2 H20)+2 (3 SiOa . Al203)+Aq) 

or a hundred others. What is there to prove that the 
silica is distributed into three principal groups, that the 
magnesia forms two distinct combinations and that the 
water occupies two different places? (I believe that I 
am mistaken and that the water occupies one and twenty 



THE THEORIES OF CHEMISTRY. 123 

places.) Does this salt by its reactions split up into a 
magnesian silicate on the one hand and an aluriiino- 
magnesian silicate on the other, etc. ? 

"I have often read the discussions which have taken 
place on this subject, and I avow that I have never found 
anything but what was either arbitrary or according to 
routine. ... If from mineral we pass to organic 
chemistry, we shall see that in this division the arbitrary 
reigns supreme. Here also are reactions appealed to as 
a means of discovering the arrangement of atoms, but, 
like the ancient oracles, one single reaction serves two 
purposes, proving both the 'for' and the 'against.' 

"As an illustration of the disorder that prevails in 
organic chemistry, I need merely state that acetic acid, 
one of the simplest of the acids, has been represented as 
the result of the arrangement of atoms as follows : 

C4HGO3 . H2O (CaHeO) CO2 . H2O 

C4H604 . H2 C4H2 . 04 

C4H60 . 02H20 C2H4 . 02 

(CHe) C2O3 . H2O C4H6O2 . H2O2 

(C2H.6)C204 . H2 C4H2 . O4H6 

For the support of every one of these formulas certain 
reactions have been invoked, and curiously enough, in 
several cases the reactions that are contrary to the hypo- 
theses are precisely those which have been adduced for 
its formation. Thus, the formula (C3H6)C203 . H2O rep- 
resents a combination of oxalic acid with an imaginary 
compound, C2H6; but nevertheless, we know very well 
that we cannot by any reaction demonstrate the presence 
of oxalic acid in acetic acid. It must be that for the 
maintenance of my cause, I have adduced the obscure 
hypothesis of an obsciure chemist. No ! The above is the 
hypothesis of Berzelius and of his school. 



124 THE THEORIES OF CHEMISTRY. 

"Lastly, for the discovery of the arrangement of the 
atoms in a body, chemists do not even wait until it has 
been submitted to a certain number of reactions. Its 
centesimal proposition alone suffices for them. 

''Some years back the discovery of an essence was 
announced, which essence contained CgHioOg. Without 
waiting for further inquiries it was at once set down as a 
hydrated oxide^3C3H2 . O . 2H2O. Subsequently, we were 
told that the substance was an acid, and its formula quickly 
became C18H18O5H2O. Some days after, the essence was 
announced to be an ether, and in the twinkling of an eye, 
the ether arrangement was given — C14H10O5 . O4H10O2, or 
better stiU, C204(Ci2Hio) . C4H8+H2O. From which, 
we learn that C12 and Hio are an intimate combination, 
while C2 and O4 are united in an ordinary manner; that 
C2O4 copulated with C12H10; that C204(Ci2Hio) is conjoined 
with C4H8; and lastly, that the whole forms a 'marriage 
of convenience' with H2O. 

"Have we at last, I will not say a rule, but even a con- 
vention which can determine in this way the arrange- 
ment of atoms? No! Every chemist follows his own 
particular course and changes his formulas as often as he 
obtains a new reaction. We shotild arrive at results just 
as satisfactory by putting the atomic letters of a formula 
into an urn and then picking them out haphazard and form- 
ing dualistic groups. 

"Next to reactions, chemists thought they had dis- 
covered in the galvanic battery a sure means of discover- 
ing the arrangement of atoms, but here we perceive an 
almost incredible example of the influence of the theories 
upon our appreciation of facts. 

"Among the thousand reactions produced by the bat- 
tery, this much is certain that there is not a single one 



THE THEORIES OF CHEMISTRY. 125 

which can be adduced in support of dualism. Neverthe- 
less, chemists who adopt this system have foimd means 
by their own interpretations to persuade themselves 
that all these various reactions prove that salts are com- 
posed of acid and oxide. The experimental proof is 
made daily in every course in chemistry. I was present 
lately at the lecture of a learned professor. Before him 
was placed a U-tube containing a solution of potassiimi 
sulphate colored with Htmus. After having passed an 
electric current through this tube, and having pointed 
out that one of the limbs had become red and the other 
blue, the professor proceeded nearly in these words: 
* You perceive in the most obvious manner that the sul- 
phate has been decomposed; the red color of this limb 
proves to you that the sulphuric acid is transported to 
this side, whilst the oxide of potassiimi is carried into the 
other limb, which is consequently colored blue. Now, 
gentlemen, all salts are decomposed in the same manner, 
but nevertheless, in the presence of this glaring fact you 
will meet with persons who venture to deny in the sulphate 
the existence of stdphuric acid, or potash, and to reject 
the system of dualism.' 

"What would the learned professor have said if one 
of his auditors had arisen and replied: *By saying that 
all salts are decomposed in this manner, your intention 
is, without doubt, to teach us that such is the case with 
the great majority, but for my part, I think the contrary. 
I think that there is not a single one which is decomposed 
in the manner you describe, not even the one that is under 
our eyes. You yourself know very well that the hypo- 
chlorites, the chlorates, the perchlorates, the bromates, 
the nitrates and the salts of gold, platinimi, manganese, 
lead, etc., give results differing extremely one from an- 



126 THE THEORIES OF CHEMISTRY. 

other, and very different from that which you have just an- 
nounced to us. With regard to the tube which is before 
us, it contains two substances, water and sulphate of 
potash, and at this moment one of the branches con- 
tains, not as you say sulphuric acid, SO3, but instead, 
hydrogen sulphate, or rather, hydrogen potassium sul- 
phate; in the other branch is found, not oxide of potas- 
sium but hydrate of oxide of potassium. So that imder 
the influence of the electric current, there results, not a 
simple decomposition, but a double decomposition.* 
I remark, moreover, that this double decomposition occurs 
similarly when potassium chloride is decomposed in the 
presence of water. . . . Here are the facts but with- 
out hypothesis. By making use of hypothesis, however, 
we can prove anything we choose. We will first prove, 
with Davy, that potassium sulphate contains K2 . SO4. 
We therefore see that in reality, under the influence of the 
battery, SO4 and K had separated from one another, and 
potassium, in the presence of water, decomposes it form- 
ing a hydrate, and the hydrogen of the water takes the 
place of the potassitim which was set free. 

"Let us now prove that Longchamp's theory is correct. 
We shall see that in reality, sulphuric acid separates from 
binoxide of potassium, and that under the influence of 
these two bodies an atom of water is decomposed. The 
oxygen unites with another atom of water to form the 
binoxide of hydrogen, v/hich combines with the sulphurous 
acid to form hydrated sulphuric acid, while the hydrogen 
unites with the binoxide of potassium to form the hydrate 
of potassiimi. We could prove quite as easily that the 
sulphate of potash consists of SK2 . O4 or S . O4 . K2. 
Nay, more ! Replace in your tube the sulphate of potash 
by sulphate of copper and at your pleasure you can cause 



THE THEORIES OF CHEMISTRY. 127 

copper or oxide of copper to be decomposed at the nega- 
tive pole, that is to say, you can at pleasure prove either 
the 'for' or 'against.' 

" Lastly, we may say that it is not the salt at all, but the 
water that is decomposed by the action of the Galvanic 
current, and that the nascent oxygen and hydrogen decom- 
posed the sulphate, the oxygen seizing upon the potassium 
and the hydrogen supplying its place. 

"Let us, however, dismiss the battery and its reactions, 
because they can teach us nothing concerning the consti- 
tution of salts. What we have said above does not prove 
that sulphate of potash is not a compoimd of acid or oxide, 
but simply that we do not know what is the arrangement 
of the atoms in the salt." 



After Berzelius proposed the theory of copulas, Ger- 
hardt stated a theory in which he recalled the "radicals" 
but also followed the unitary notion contained in Dumas' 
older type theory. This he called the "theory of 
residues." It soon yielded to a newer and more satis- 
factory one. Neither Laurent nor Gerhardt was satisfied 
with the existing methods used in classifying compounds; 
and together they made the deductions upon which they 
based a new theory — the ''type theory." 

There is a wonderful and very intimate connection 
between the "radical theory," the "substitution theory," 
the "nucleus theory," the "older type theory" and this 
"new type theory." In the new type theory, there is 
unquestionably the idea of unity. Gerhardt declared 
that the dualistic idea had pertained, consciously or 
unconsciously, to some degree at least in all of the theories 
preceding Dum.as' older type theory. 



128 THE THEORIES OF CHEMISTRY. 

It becomes evident upon examination of this last 
proposition, that Laurent and Gerhardt were seeking 
light. Finally, the type theory was evolved by them. 
The two men were satisfied because they had found 
that for which they had been seeking, it was unity. 
This idea was the core of their type theory. There 
was also simplicity in this new theory, because around 
a few simple unitary types could be grouped all the 
complexities which had existed in the past, and simplicity 
was desirable. This theory, next to that of phlogiston, 
dominated chemistry the longest time. 

The master mind of the type theory is Gerhardt's. 
Laurent and Gerhardt proposed that compoimds should 
be arranged in series, each series being comparable to a 
type. These types were of various classes. The hydrogen 
type, from which might be derived compounds like HCl 
and allied substances; this type also included the hydro- 
carbons. The water type and the ammonia type 

jj jj H 

^ HYDROGEN TYPE „0 WATER TYPE HN AMMONIA TYPE 

H H jj 

Hydrochloric acid instead of being viewed as a derivative 
of the hydrogen type, was considered by some as a dis- 
tinct type from which was derived chlorides, etc. The 
water type included the hydroxides, NaOH, KOH, etc., 
and most of the organic compounds. The ammonia 
type included a number of bodies, the amines, etc. Sul- 
phuretted hydrogen was an auxiliary type yielding the 
sulphides. 

This theory was criticised in that it still left the 
question concerning the formation of organic compounds 
unanswered. 



THE THEORIES OF CHEMISTRY. 129 

Kekule, in 1857, extended the type theory. He origi- 
nated "mixed types." They were obtained by joining 
together two of the simple types : 





H 


H 


HN 


H 


H 


^0 


lo 



These mixed types were used for the explanation of those 
compoimds which could not be compared to the simple 
types. In this same year, Kekule proposed an additional 
type — the methane or marsh gas type (CH4). Many of 
the organic compounds were considered comparable to 
this type. But prior to this proposal of Kekule came the 
assertions of Kolbe and Frankland. 



Before 1857, Kolbe proposed a "new radical theory," 
in which he stated, "that the elements within the radical 
could be replaced by other elements. Organic com- 
pounds are all derivatives of inorganic, and result from 
the latter — ^in some cases directly — by wonderfully simple 
substitution processes . ' ' 

Almost simultaneously with the preceding statements 
came this from Frankland, " that the copulation of radicals 
with elements depended upon a property inherent in the 
elementary atoms." 

Kolbe agreed with Frankland's statement. Later, he 
wrote: "the combination of elements into radicals and 
into their various compounds is due to a property of the 
atoms of elements. This property is inherent in the atom. 
It is called the saturating capacity of the atom." 



130 THE THEORIES OF CHEMISTRY. 

It had not been possible for chemists to explain why 
elements formed the types until Frankland, and more 
especially Kolbe, brought this "property of atoms" to 
their attention. 

Kolbe continues, "The hydrogen atom has a saturating 
capacity of one, and the chlorine atom has a saturating 
capacity of one. They combine and form the type HCl. 
In the water t3rpe, two hydrogen atoms and one oxygen 
atom are united; that combination must be due to a 
saturating capacity on the part of the oxygen atom equal 
to two, whereby it is able to hold the two hydrogen atoms. 
In the ammonia type, there is a trivalent atom of nitrogen 
which has the power to hold three hydrogen atoms or 
three atoms of any other element having a saturating 
capacity of one. In the methane type, it is the saturating 
power of carbon, which gives rise to the class of bodies 
which can be constructed about a carbon atom or any 
other atom which has the power of saturating equal to 
four hydrogen atom^s." 

Frankland, at this time expressed his views as follows: 
"When the formulas of inorganic chemical compoimds are 
considered, even a superficial observer is impressed with 
the general symmetry of their construction. The com- 
pounds of nitrogen, phosphorus, antimony and arsenic, 
especially exhibit the tendency of these elements to form 
compounds containing three or five atoms of other ele- 
ments; and it is in these proportions that their affinities 
are best satisfied. ... It is sufficiently evident, 
. . . that such a tendency or law prevails, that, no 
matter what the character of the uniting atoms may be, 
the combining power of the attracting element, if I may 
be allowed the term, is always satisfied by the same 
nimiber of these atoms." 



THE THEORIES OF CHEMISTRY. 131 

Kolbe and Frankland are iindoubtedly the originators 
of the idea of "valency." The attempt has been made in 
all subsequent writings upon this subject to ignore these 
two chemists and their ideas concerning the "saturating 
capacity" of the elementary atoms. 

Kekule disagreed with Kolbe's utterances. In his 
judgment the properties of compoimds were not to be 
regarded as due primarily to the saturating power of the 
atoms of the various elements present in such compounds. 
At the same time (1857) as has been mentioned, he advo- 
cated the use of the marsh gas or methane type, giving to 
it the formula CH4. Many chemists have ignored Kolbe 
in ascribing to Kekule the discovery of the fact of the 
quadrivalence of the carbon atom. Later, Kekule explained 
the combination of two carbon atoms, in this combination, 
he believed that each carbon retained its normal valence, 
two of the four affinity units of each carbon atom being 
neutralized in holding the carbon atom.s together. This 
suggestion finally led to the doctrine of the linking of car- 
bon atoms. Kekule then intimated that "a more compact 
combination of carbon atom.s" might be assumed to be 
possible. 

The ideas of dualism and of copulas were passing and 
other theories were being formulated to replace them, 
and these succeeded, chiefly, because they w^ere not too 
systematic. This idea of the "saturating capacity of 
atoms," proposed by Kolbe and Frankland and later by 
Kekule and independently by Couper, gives liberty to 
chemists to formulate compounds guided by one thing 
only, namely, that the atom possesses a definite saturating 
power. 

Kolbe suggested an additional thought, "that a com- 
pound derived from the 'water type,' possessing the 



132 THE THEORIES OP CHEMISTRY. 

properties of hydroxides; or a compound derived from 
the 'ammonia type,' and possessing basic properties 
does not owe its character to the type from which it is 
derived, but obtains its characteristics from its component 
elements. The elementary atoms are to be regarded as 
the true units of a compound." 

Couper developed the idea of the "saturating power of 
the atoms of elements" independently of Kekule. In 
referring to Gerhardt's development of types, he objected 
to the vagueness of that idea as a means of classification, 
and especially opposed Gerhardt's opinion that the molec- 
ular constitution of bodies could never be ascertained 
by chemists. "Wovdd it not be rational," says Couper, 
*'in accepting this veto, to renounce chemical research 
altogether?" Couper believed that this dictimi of Ger- 
hardt was to be traced to the excessive employment of 
compound radicals, and to Gerhardt's ideas concerning 
the properties of these radicals. Gerhardt ignored the 
fact that these radicals can have no properties that are 
not a direct consequence of the properties of the individual 
elements of which they are composed, and hence, ascribed 
to them an unknown ultimate power which cannot be 
explained. 

Chemists are, now, prepared to say with Kolbe, Kekule, 
Couper and others of this time, that this so-called ** saturat- 
ing capacity" which Couper terms "chemical affinity" 
is a property inherent in and common to the elementary 
atoms. 

Couper and Kekule were so impressed with this idea 
of "affinity" that they distinguished between "affinity 
of kind" and "affinity of degree." Couper cited, as 
examples, the two oxides of carbon, CO and CO2; — the 
former, he thought, expressed "affinity of kind," and the 



THE THEORIES OF CHEMISTRY. 133 

latter expressed "affinity of degree" — the last degree — 
the ultimate affinity or combining power of the element 
carbon. 

If there be a difference between the teachings of Couper 
and Kekul^, it is that Kekule discriminated more clearly 
than Couper, between the "affinity of kind" and the 
"affinity of degree," or between chemical affinity and 
basicity of atoms. He clearly distinguished, and this 
has been too much forgotten in the later development of 
chemical affinity, between the "equivalent weight of the 
elements" and the "equivalency — saturating power — 
combining power of the elementary atoms." 

Kekule showed that this new theory, presented by him 
and Couper, was dealing with definite entities called atoms. 
These atoms or entities possessed definite properties. 
He did not deal with their unit weights. He was only 
concerned with the combining or substituting power of 
the atoms and he proposed that this power of the 
atoms be made comparable to the atom of hydrogen 
taken as unity. He demonstrated to his own satisfaction 
that the carbon atom was quadrivalent towards hydrogen. 
Further, that two atoms of carbon did not combine 
with more than six atoms of hydrogen; three with more 
than eight; four with more than ten, and so on. 

Kekule's paper, in which these thoughts appeared, was 
published, in 1858, in voltime 106 of the "Annalen der 
Chemie." It is a contribution deserving very careful 
perusal and thought on the part of every student of 
chemistry. In that paper, Kekule, not only called atten- 
tion to the quadrivalence of the carbon atom, but he also 
laid the foundation stone of the modem hypothesis of 
atom linking. 

Kekul^ and Couper insisted that if a definite conception 



134 THE THEORIES OF CHEMISTRY. 

of the connections between the properties and the structure 
of compounds is to be obtained, it must be based upon 
the study of the combining power, the saturating power, 
the valency of the elementary atom; for, remarks Couper, 
"the whole is simply a derivative of its parts." 

Although the ideas of "radicals" and of "types" had 
been dismissed, Kekule and Couper retiim in their dis- 
cussion of "valency," to the four fundamental types: 
hydrogen or hydrochloric acid, water, ammonia and the 
methane type. With these as a basis, they developed 
the fundamental thoughts of their "theory of valency." 
In the first type (H2), one atom of hydrogen is united 
with a second atom of the same element, in (HCl), one 
atom of chlorine is in union with one atom of hydro- 
gen; in the second or water type (H2O), one atom of 
oxygen is combined with two atoms of hydrogen; in 
the third, or the ammonia type (NH3), one atom of 
nitrogen is in union with three atoms of hydrogen; and 
in the marsh gas type (CH4) one atom of carbon is in 
union with four atoms of hydrogen. 

How is the "valence" of the elementary atom to be 
expressed? The atom of chlorine, with reference to 
hydrogen as the standard, has the saturating capacity, the 
combining power, the affinity, the valency of one; the 
oxygen atom has a combining power, a valency of two 
compared with hydrogen ; the nitrogen atom has a valency 
of three compared with hydrogen ; the carbon atom has a 
saturating power of four. These atoms, chlorine, oxygen, 
nitrogen and carbon respectively are, as they were then 
said to be, univalent, bivalent, trivalent, quadrivalent. 

A bivalent atom is one which combines with not more 
than two atoms of chlorine, bromine, or fluorine, and is, 
therefore, equivalent to another atom which combines 



THE THEORIES OF CHEMISTRY. 135 

with not more than two atoms of hydrogen. In the same 
manner, trivalent and quadrivalent atoms are defined. 
When it is said that one atom is combined with a certain 
number of standard univalent atoms, direct interaction 
betv/een these atoms in the molecule is assumed. When 
one atom of bismuth is combined with three atoms of 
chlorine, each of which is univalent, it is implied that in 
that molecule of bismuth chloride direct action of some 
kind occurs between the one atom of bismuth and the three 
atoms of chlorine. In marsh gas, one atom of carbon is 
in union with four imivalent hydrogen atoms and it is 
assumed that in that gaseous molecule there is direct 
action and reaction of some kind between the one atom 
of carbon and the four univalent hydrogen atoms. 

These points are famiHar to all who have studied chem- 
istry, and they may seem to receive unnecessary attention, 
but it must be remembered that the development of a 
chemical theory is being followed; and, hence, the aim 
must be to piu-sue the line of thought as developed by 
Kekule and Couper. 

What term shall be used to express the idea of the 
** saturating capacity of the atoms?" Affinity, equi- 
valency, atomicity, saturating power, valency have been 
employed. "Valency" is to be preferred since it suggests 
no idea to which exception could be made. The terms, 
univalent, bivalent, trivalent, quadrivalent, quinqui- 
valent, etc., have been developed and their application 
to atoms indicated. 

How, then, are the atoms of chlorine, bromine, iodine 
and hydrogen equivalent? Kekule did not imply, how 
much by weight of the elements was to be looked upon as 
equivalent. He emphasized, in all his writings upon this 
subject, that "eqmvalency" and "equivalent weight" 



136 THE THEORIES OF CHEMISTRY. 

were not interchangeable terms. How, then, are these 
atoms equivalent? They are equivalent in this respect, 
that each combines with one and only one atom of 
hydrogen to form a gaseous molecule, for example, one 
atom of chlorine combines with one atom of hydrogen to 
form the gaseous molecule, hydrochloric acid gas, or one 
atom of hydrogen combines with a second atom to form a 
gaseous molecule of hydrogen. These atoms are equivalent 
and are to be regarded as standard imivalent atoms. 

The valency of the atoms of other elements can be 
determined by imiting them with these standard uni- 
valent atoms, and then determining how many atoms 
of the standards are present in the molecules of the 
products. It must be understood, hov/ever, the valency 
of the atom of an element cannot be determined accu- 
rately except at least one gasifiable com.pound can be 
prepared, composed of a single atom of the element in 
question combined with these standard univalent atoms 
and with such atoms only. For example, the valencies 
of copper and gallium cannot be determined imless gasifi- 
able molecules of copper or of gallium and some standard 
univalent atom can be prepared. This accomplished, 
the valency of the copper and gallium will be the maximum 
nrnnber of atoms of hydrogen, of bromine, of chlorine, 
etc., with which the copper or gallium atom combines to 
3deld gaseous molecules. A step further, the valency of 
an atom is a number which expresses the maximum 
nvimber of standard univalent atoms, between which and 
the specified atom there is direct interaction in a gaseous 
molecule. This definition gives a working hypothesis 
which can be satisfactorily used in arriving at knowledge 
concerning the arrangement of molecules. 

Can any definite meaning be drawn from the expres- 



THE THEORIES OF CHEMISTRY. 137 

sion, the carbon atom is quadrivalent or the carbon atom 
has four valencies? 

"The statement that an atom of carbon has four 
valencies, four units of affinity, cannot mean that the 
force of chemical attraction of a carbon atom is divided 
into four parts within the atom, the force has no meaning 
apart from two or more reacting bodies and here it is only 
one. Force is a name given by one of the parties to a 
transaction, but a transaction involves, at least, two 
transacting parties. But force between the carbon atom 
and another atom must vary with the external conditions, 
probably with distance, with the mass, and the chemical 
nature of both atoms. 

"Then to give another view of it, the carbon atom has 
four valencies, four units of affinity. This cannot mean 
that four parts of the carbon atom are chemically active 
and the other parts inactive, such an hypothesis leads, 
at present, to contradictions. Moreover in the present 
state of our knowledge, it is inadvisable to hazard 
hypotheses as to the inner structure of atoms in order to 
explain these peculiar chemical phenomena or changes. 
Indeed, the atoms may not be homogeneous, but at 
present they are the ultimate particles to be considered 
in chemical changes. 

"Again, the expressions under consideration cannot 
mean that the chemical energy of the carbon atom is 
divided or is always divisible into four parts. What is 
to be the unit of chemical work? The mass of matter 
fixed by a given atom? Where then is the equivalency 
between one atom of oxygen with the mass sixteen and 
two atoms of chlorine with the mass seventy-one? Let 
a carbon atom combine with foiu* hydrogen atoms, — 
the total chemical energy of that carbon atom vanishes. 



138 THE THEORIES OF CHEMISTRY. 

Let a carbon atom combine with two atoms of oxygen, — 
the total chemical energy of the imiting atoms again 
disappears. But if the carbon atoms possess four imits 
of affinity, the oxygen atoms two units of affinity, and the 
hydrogen atoms one unit of affinity, the heats of formation 
of the compound molecules respectively ought to be 
equal. But the differences between the heats of formation 
of carbon compounds show that the expression 'carbon 
has four units of affinity' cannot mean that the chemical 
energy of the carbon atom is divisible into four parts, 
unless indeed the imit of affinity is variable and is varied 
for each combination of carbon with other atoms. 

"The carbon atom has four valencies. Can this 
mean that the atom exercises force in four directions? 
The so-called valency then becomes a direction. But there 
is no force exercised until the mutual atomic transaction 
begins; the carbon atom considered alone has therefore 
no valencies. In the molecule CO: force is exercised by 
the carbon or by the oxygen atom; but the remaining 
valencies are sometimes said to be mutually satisfied. 
That is based on the present hypothesis, the carbon atom 
in the molecule CO exercises force in two directions on 
itself. But there again is the hypothesis of the non- 
homogeneity of the carbon atom and the existence of 
active and inactive parts in that atom. Further, in the 
vibration of a carbon atom there are four points, at each 
of which mutual action can occur between this atom and 
another atom. On this supposition, a double link would 
mean that mutual action occurs between the two atoms 
thus linked at two of these positions. The formula OCO 
would mean that in performing a vibration, the carbon 
atom acts twice and is twice acted upon by each oxygen 
atom. But if so, surely a double link would imply 



THE THEORIES OF CHEMISTRY. 139 

molecular stability, whereas, it frequently means the 
reverse." 

While a clear physical conception cannot be formed of 
the meaning of the phrase "the carbon atom has four 
affinities, foiir bonds," and although such formulas as OC, 
OCO, H2CCH2 and HCCH which spring from the notion 
of bonds, fail to call up in the mind clear images of 
the things they are meant to represent, nevertheless, it 
may be urged that inasmuch as the properties of such 
molecules show that chemical functions of the atoms of 
carbon vary in different molecules all of which are com- 
posed of carbon and hydrogen only, or of carbon and 
oxygen atoms only, it is convenient to express such varia- 
tions of function in our nomenclature, and the expres- 
sions single, double, treble linked carbon atoms, are 
convenient for that purpose. The expressions single, 
double and treble linkings, mutual satisfaction of imits 
of affinity, and the like, assume a knowledge, which at 
present is not possessed. 



Is the valence of any one atom fixed or does it vary? 
To vary the valencies of atoms without proof for so doing, 
causes endless confusion and the applications of this 
hypothesis of valency become amusing exercises of fancy. 
The chlorine atom has the combining power or valence of 
one. Does this standard univalent atom ever present 
itself in a different role ? Chlorine is a imivalent atom 
compared with hydrogen; generally it is a univalent 
atom compared with oxygen, but an oxide of chlorine 
exists, having the formula CI2O3, the derivatives of this 
oxide have been studied. 

To satisfy the demands of the thought of ^ fixed valence. 



140 THE THEORIES OF CHEMISTRY. 

this oxide must be graphically represented Cl-0-O-O-Cl, 
but many of the properties of this oxide do not permit 
a formulation of this description. Hence the question 
is urged, does the chlorine atom always possess a valency 
of one? Some of the compoimds of iodine cannot be 
explained by maintaining the idea of a fixed valence for 
that element. As the elements are more thoroughly 
studied reasons for admitting the widening of the idea 
in regard to the saturating capacity of the atoms becomes 
more and more evident. The halogen atom is univalent 
when compared with the standard hydrogen, but when 
compared with oxygen, another saturating power is 
manifested. An atom of phosphorus compared with 
hydrogen is both trivalent and quinquivalent; the iron 
atom in its simplest combination with chlorine is bivalent, 
but with the same element it shows a higher combining 
power of three. Valence then is not fixed, it varies. 

The chemists, who lived in the 60 's were, of course, 
guarded in all their speculations concerning the com- 
binations of atoms. They natiurally felt, that if the prop- 
erties of compounds were to be attributed to the nature 
of the atoms, then the saturating power must be a fixed 
value. They would not entertain any other idea. 

Is it then surprising that at the present time students 
of chemistry have difficulty in convincing themselves of 
the variableness of valency? At first, it was taught that 
the valency of elements was as fixed as the laws of the 
Medes and Persians; to abandon that idea was to give 
up one of the articles of faith. The elements were arranged 
in groups with atoms of equal valency, as monads, dyads, 
triads, etc.; later, they were divided into two groups — 
those with even valencies and those with odd valencies. 
An odd valence could never change to an even, and an 



THE THEORIES OF CHEMISTRY. 141 

even could never become an odd valence. Things have 
undergone a change since then. The early opinions in 
regard to the fixedness of this combining power of the 
atoms were modified. At first it was reluctantly admitted 
that valence may vary. But how? The elementary 
atom which had a saturating capacity of one, if it varied 
in its combining power or satiirating capacity, became 
an atom with a satiu^ating capacity of three, five, seven. 
The atom which had a valence of two, if it changed, 
became an atom having a valence of four, or six. Hence, 
the elements were arranged into two grand divisions: 
the artiads, those of even valencies, and the perissads, 
those of odd valencies. As time rolled on the article of 
faith was again modified. It was discovered that valency 
did not only vary from odd to odd and from even to even, 
but it varied from odd to even and even to odd. The 
valency of the iron atom changes from that of an artiad 
in ferrous chloride, to that of a persissad in ferric chloride, 
or from an even to an odd valence. 

At the present time there are chemists who admit 
that the valency of an atom, its combining power, its 
affinity (using these terms in the sense in which they were 
used by Kekule and Couper) not only varies — ^not only 
varies from odd to even and from even to odd, but 
further, that valence is and may he anything— ^.e^ending 
upon the conditions imder which the atoms of the elements 
happen to be placed at the time of their action and 
reaction. This is the most radical assertion that has 
been made during the years witnessing the development 
of the idea of valence, and yet chemists, some chemists, 
beHeve that it expresses the truth. 

Examine the formula of arsenopyrite, FeSAs. Other 
examples from the mineral world might be placed with 



142 THE THEORIES OF CHEMISTRY. 

this one. It is noticeable that the examples showing these 
peculiarities in valence are not found in the organic world; 
carbon, in its ramifications, combines principally with 
oxygen, nitrogen and hydrogen, and exhibits an invariable 
valence. 

The valencies of atoms are graphically expressed in 
different ways. They were formally written, and even 
to-day may be seen written, in this manner, CP, O", or 
with lines to left and right, as CI — , — O — , = C = 0. 
Then followed an unconscious connection of symbols 
by means of lines, strokes, points, hooks or what not; 
so that compounds were looked upon as consisting of 
atoms hooked together. 



The idea of units of affinity, or valency has been carried 
too far. It is very probable that the valency of the 
elementary atoms may be found to vary periodically 
with variations in the relative weights of these atoms. 
If this general statement is thoroughly established, the 
exact nature of the periodic function determined and the 
true atomic weight for each element fixed, then deductions 
may be drawn from the periodic law concerning valency. 

Valency has been developed principally through the 
study of carbon and its derivatives ; but as other elements 
and their combinations were examined difficulties arose. 
In looking over a broader field than that which was 
visible to the chemists of 1858 and the succeeding years, 
many facts were discovered not in harmony with the 
pronunciamento of Kekule. The ideas concerning valence 
have changed until little remains of the original theory. 
What is the next step ? 

It must be remembered that these theories are merely 



THE THEORIES OF CHEMISTRY. 143 

working hypotheses. Just as soon as they become 
inadequate, or fail to account for the phenomena being 
observed, they are laid aside and new theories take their 
place. And so with "valence." 



Explanation of the Constitution of Chemical 

Compounds by Means of Physico-Chemical 

Processes. 

development of physical chemistry. 

The fact that an intimate relation existed between 
the chemical and physical properties of substances first 
appealed to the chemists of Lavoisier's time. Since 
then, investigators have sought to trace this relationship 
more completely. The laws of Avogadro and of Gay- 
Lussac are evidences of the investigations along physical 
lines; as are also the deductions of Dumas — determi- 
nation of vapor density; of Dulong and Petit — law of 
specific heats; and of Mitscherlich — law of isomorphism. 

These later chemists made use of physico-chemical 
methods in their researches, but, it remained for Her- 
mann Kopp to be the real pioneer in the field of physical 
chemistry, in the discovery and use of physico-chemical 
processes for the determination of the constitution of 
chemical compounds. 

In 1850, Kopp proposed a method whereby the "specific 
voltmie" of liquids coiild be determined. This investi- 
gation marks the beginning of physical chemistry. Specific 
volume was found by dividing the molecular weight by the 
specific gravity of the liquid. This discovery of Kopp is 
not epoch-making, but it is one step forward in the search 



144 THE THEORIES OF CHEMISTRY. 

after truth regarding the constitution of chemical com- 
pounds. He observed that in many substances the dif- 
ferences in specific volimies were proportional to the 
differences in the structure of the compounds; and that 
the specific volumes of isomeric bodies were different. 
Those of the normal hydrocarbons, which were deter- 
mined at the melting points, approached with an 
increase of carbon to a definite limit; and in the ho- 
mologous series, he observed that for each additional 
CH2 there was an increase of about twenty-two imits 
in specific volume. Kopp thought it possible from such 
data to deduce from the molecular value of compotinds 
the atomic volume of their constituent elements. He 
foimd eleven (11) to be the value for carbon, and 5.5 
for hydrogen. Varying values w^ere obtained for oxygen 
and nitrogen, the value apparently being dependent 
upon the manner in which the elements were combined. 
When oxygen was joined to carbon by a double bond 
in any chemical compound, it had a volume of 7.8; but 
when it was joined by a single bond it had a volume of 
12.2. Kopp gave two values for sulphur, 12.2 and 20.6, 
the lower value being assigned to sulphur when it con- 
nected two elements; and the higher being assigned to it 
when it was saturating. He also discovered that meta- 
meric liquids generally possessed the same specific volume. 
It is evident, if the observations of Kopp are correct, 
that early in the sixth decade of the nineteenth century, 
chemists were beginning to find means by which they 
could determine the constitution of an organic compound. 
It was possible, from the determination of the specific 
volume of a substance containing oxygen, to state whether 
that compound was an alcohol or a ketone. This was 
an advance in chemical knowledge. 



THE THEORIES OF CHEMISTRY. 145 

Unfortunately, the observations of Kopp were per- 
mitted to go unnoticed for many years. Indeed, they 
received but little attention until Lessen gave them 
careful consideration. He conducted experiments along 
lines described by Kopp, and obtained for each increment 
of CH2, in a series of monobasic acids, a value agreeing 
very closely with the constant first published by Kopp. 
In the primary alcohols, Lossen foimd that each additional 
CH2 gave a numerical variation of from 18.7 to 21. He 
studied the propyl, isopropyl and allyl compoimds and 
reported that in normal propyl and allyl compounds the 
CH2 caused a variation of 5.5 to 5.9. 

Later, Thorpe studied quadrivalent carbon atoms and 
their values in compoimds, substituting chlorine in many 
of these for hydrogen, and discovered that the values 
for chlorine were, likewise, variable; he concluded, from 
his experiments, that the specific volume and also the 
boiling point of a hydrocarbon were changed by each 
atom of chlorine added to the compound. 

From the studies of Thorpe and Lossen, the conclusion 
was reached that Kopp's deductions were only approx- 
imately true. They foimd, contrary to Kopp's assertions, 
that many isomeric bodies had different molecular values 
in unsaturated compoimds. Each carbon doubly linked 
to carbon increased the molecular volume. A compound, 
containing carbon atoms in the form of a ring, possessed 
a smaller molecular volume than the isomeric bodies 
with an open chain, in which the carbon atoms were 
linked together by double bonds. Thorpe ^nd Lossen 
demonstrated that the specific volume of substances was 
influenced by the nature of the linking of the atoms. Even 
when their atoms were similarly linked, isomeric bodies 
possessed different molecular volumes. Ethylene chloride 



146 THE THEORIES OF CHEMISTRY. 

and ethylidene chloride show a difference of 47 in their 
molectdar values, from which it follows that the moleciilar 
voliimes of organic bodies are apparently conditioned 
by two influences: the one of an additive nature, which 
was observed by Kopp, the other of a constitutive nature, 
discovered by Thorpe and Lessen. After these two 
investigators had brought Kopp's deductions before 
chemists, others began to study his writings. 

Kopp's experiments were conducted under conditions 
less favorable than those which a chemist of to-day meets 
in carrying out investigations of a like character. 

Kopp was also the first chemist to point out the existing 
connection between the boiling point and the composi- 
tion of compounds. He writes in his papers that he com- 
pared substances at their "boiling points." There is, 
however, no reason to believe that he corrected these for 
the lattitude in which he worked or for the pressure at 
which they were taken. 

The results of Lessen and Thorpe differ from those of 
Kopp principally because the methods pursued by him 
were not strictly followed. He always used a peculiar 
style of distilling flask, and the bulb of the thermometer 
was placed on a level with the opening of the delivery tube 
and at the same time at a definite distance above the 
boiling liquid. The result will be different, if the ther- 
mometer bulb is in any other position or if the delivery 
tube is exposed to the air currents in the room. Kopp 
made no attempt to shield the delivery tube from the air 
currents in the room. The apparatus used by him was 
very different from that obtainable to-day. Thorpe and 
Lossen worked with entirely different and more refined 
instruments. 



THE THEORIES OF CHEMISTRY. 147 

Before Kopp's discovery, an investigation of a com- 
pound gave only the weight of the elements in the com- 
poimd in per cent, and this was the only data possible to 
obtain. Thereafter, the manner in which the elements 
existed in that compoimd could be determined. The 
determination of the specific volume indicated the pres- 
ence of the hydroxyl group, made evident that all of 
the hydrogen of an alcohol was not alike — that one 
hydrogen atom was beyond the radical, and disclosed 
the condition — ^the linking — of the carbon in the com- 
poimd. 

Kopp's work possessed value; and a description of it 
constitutes the first chapter in physical chemistry. 

Kopp sought to determine the specific volume of the 
natural sulphides, but his results led to no definite con- 
clusion. Whether his investigations in this direction will 
be helpful in the solution of the molecular magnitude of 
these minerals remains to be seen. Every student of 
chemistry will find much that is instructive in Kopp's 
early papers, and they merit careful reading. They are 
to be foimd in the *' Annalen der Chemie," Vol. 46, p. 212, 
Vol. 96 and succeeding volumes. 



Some years after the preceding discovery was made, 
it was ascertained that a determination of the ** molecular 
refraction" of substances gave valuable information con- 
cerning compounds. This was also a physico-chemical 
method. The results were interesting and demonstrated 
that a relation existed between the "optical behavior of 
solid and liquid substances and their chemical composi- 
tion" between the composition of compounds and their 



148 THE THEORIES OF CHEMISTRY. 

"refractive indexes." The co-efficient of refraction — the 
refractive index — the refractive quotient — ^for homogene- 
ous light, passing from one medium to another, represents 
the ratio of the propagating velocities Vi and V2 in both 
media. 

V2 

For single refracting media (in which similar optical 
deportment is observed in all directions) N is independent 
of the directions of the incident and refractive angles, 
therefore 

>T Vi sin I 



V2 sin R 



a constant number for light of a definite wave length. 
The specific refractive power or refraction constant 



N2-1 



(N2+2)D, 



in which Dt is the specific density of the liquid for the 
temperature t degrees and since 



Sin R 

for light of definite wave length, then the molecular 
refraction for a particular substance will be found by- 
using the formula : 

(Ng-i)M 
~ (N2+2) D, 



THE THEORIES OF CHEMISTRY. 149 

the refractive power multiplied by the molecular weight of 
the substance. Moleciilar refraction is nearly the same 
in the case of compounds with isomeric positions, but it 
is dissimilar in the case of isomerides where the saturation 
of atoms is different, for example, in acetone and in allyl 
alcohol. Fortimately, the molecular refraction of a 
compoimd is approximately equal to the sum of the atomic 
refraction of the elements which compose it. The molec- 
ular refraction of a liquid carbon compound is equal to 
the sum of the atomic refractions. 

At first it was believed that only one atomic refraction 
existed for each element in its compounds. 

Landolt demonstrated that substances having the same 
refractive equivalents were probably the same. He 
writes: "The refractive equivalent depends upon the 
nimiber of atoms in the molecule rather than on the 
arrangement of the atoms." 

The refractive indexes of compoimds were compared 
and it was foimd that the position or the linkage of the 
carbon atoms, as well as the hydrogen and oxygen present, 
caused differences in the molecular refractions. The 
refractive index of methyl chloride was foimd to be 13.2 
and that of aldehyde 18.6. One carbon atom caused this 
difference. Acetaldehyde had a refractive index of 1.06, 
while the value for ethyl alcohol is 2.12; a difference of 
1.06 caused by two hydrogen atoms. Between aldehyde 
and acetic acid there exists a difference of 2.5 in the 
refractive index. 

From these and many other observations, which were 
made at the time, it was concluded that one carbon atom 
caused a difference of five (5) units in the refractive index 
if it were singly linked to another carbon atom; but with 
an oxygen atom it caused a difference of three (3). 



150 THE THEORIES OF CHEMISTRY. 

Briihl continued this subject and, later, stated "Only 
the univalent elements have a constant atomic refraction, 
while the atomic refraction of the polyvalent elements, 
like oxygen, sulphur and carbon, is influenced by the 
manner in which these elements are combined one with 
the other." The group CO increased the refractive 
index of a compound more than oxygen singly combined. 
A difference of 1.78 in the refractive index is observed for 
carbon doubly linked to carbon. It became evident that 
important facts relating to the union of atoms in the 
molecule of carbon compounds could be learned from a 
determination of the refractive index of the molecule. 

After studying benzene derivatives, Bruhl concluded 
that in the benzene nucleus three doubly linked carbon 
atoms were present. The work of Bruhl and Landolt 
was published in the " Annalen der Chemie." 

Beginning with 1850, and continuing to the present 
time, efforts have been made to arrive at the constitution 
of compounds (organic, especially) from a study of their 
physical properties. Determinations of the refractive 
index and of the specific volume of organic substances 
have been helpful in denoting the configuration of the 
molecules. The refractive index of most of the minerals 
is known. Will not this information prove helpful in 
determining the composition of these inorganic com- 
plexes? 



*' Isomorphism" was not thoroughly understood when 
Kopp was conducting his experiments. Beginning with 
the year 1819, the subject received some attention. 
In 1820, the first hydrocarbons were discovered that 
illustrated this point. In 1830, Berzelius claimed "that 



THE THEORIES OF CHEMISTRY. 151 

bodies of similar composition but possessing different 
properties were isomerides." And, in 1831, he distin- 
guished between isomerides thus: "isomerism of bodies 
of different molecular weight was due to polymerism; 
of bodies of like molecular weight to metamerism." 

The ideas concerning isomerism were not fully under- 
stood and it was felt that these should be applied with 
great caution. This field of investigation was new and 
much experimentation was required to perfect the knowl- 
edge of this branch of chemical science. These early- 
ideas were more completely explained later. The further 
study of isomeric phenomena led finaUy to the establish- 
ment of a "theory of atom linking" — a "structural 
theory of chemistry." This theory was not only instru- 
mental in explaining the cause of the innumerable 
isomerides, that had been observed before and after 
Kopp's age, but it was also helpful in enabling chemists 
to predict unknown compounds. 

Kekule had given a formula to express the saturated 
compounds of carbon — CnH2n+2. Concerning the car- 
bon compounds which were poorer in hydrogen (the 
unsaturated compounds) he favored two views, namely, 
that a more compact union had occurred (a double, or 
treble linking of the pairs of carbon atoms), or that some 
of the carbon valencies were unsaturated. He assumed 
the presence of an "open chain" in the saturated com- 
pounds, and a "closed chain" or "benzene ring" in the 
unsaturated compounds. "In the latter each carbon 
atom in the molecule is united with a carbon atom. The 
structural formula which results is a hexagon — in the 
angles are placed the carbon atoms linked alternately to 
each other by one and two bonds, and each carbon atom 
is combined in every case with one atom of hydrogen or 



152 THE THEORIES OF CHEMISTRY. 

with an atom of an element equivalent in combining 
power to hydrogen, thus : 



CH 



HC 



HC 





CH 



CH. 



CH. 



From this formula he deduced a number of facts; the 
truth of these and other asstimptions has since been 
verified. This work of Kekule is frequently regarded 
as the pioneer work in "structural chemistry." Recall, 
however, the work of Gmelin. 

Kekule 's "structural theory" thus successfiilly ex- 
plained the constitution of the so-called "aromatic com- 
poimds." Kekule, later, decided in favor of the idea 
that the carbon valencies were not all satisfied in these 
unsaturated or aromatic compounds of carbon. 



Jacob Wislicenus had studied the lactic acids; and by 
his researches had added much to advance the doctrine 
of isomerism. 

In 1860, Pasteur investigated the "optically active 
tartaric acids" and the inactive racemic acid produced 
by their combination. He thought that ' * optical activity ' ' 
was probably due to "molecular asymmetry" and de- 



THE THEORIES OF CHEMISTRY. 153 

pended, to some extent at least, upon the crystalline form 
of the substance. 

WisHcenus, in 1873, wrote, "facts compel us to explain 
the difference of isomeric molecules of like structural 
formula by stating that the atoms take up a different 
arrangement in space." Did this statement of Wis- 
Hcenus explain the existence of ** optically active" bodies? 
This subject was being investigated and, in 1874, van't 
Hoff and Le Bel independently announced that the 
cause of "optical activity" was due to the presence 
in these substances of an asymmetric carbon atom — a 
carbon atom joined to four different atoms or radicals. 

How were these atoms to be represented? They were 
arranged by van't Hoff at the comers of a tetrahedron 
having the carbon atom in the center. This idea of the 
arrangement of the atoms in space and their rotation in 
space was extended by WisHcenus. "The theory of 
stereo-chemistry — space chemistry" resulted. 

This theory has been helpful in explaining the com- 
poimds of carbon. Will it not be applicable to certain 
inorganic compounds? Tin, cobalt, titanium, molby- 
denum, sulphiu" form compounds, many of which exhibit 
"geometrical isomerism." 



Another physico-chemical method was proposed for 

the determination of the constitution of substances. 
The first attempts to determine the amount of heat 
Hberated during chemical reactions had been made, and 
some idea of the affinities of the reacting substances 
had been deduced from the results; but exact relations 
of this nature could not be formulated imtil the heat 
changes were exactly measured. The method, now 



154 THE THEORIES OF CHEMISTRY. 

proposed, consisted in exactly measuring the thermal 
changes occurring during chemical reactions and from the 
data obtained, deducing the chemical afiinities active 
in the process. This method did not accomplish all that 
was expected. 

Thermo-chemical data, are valuable in that they aid 
in ascertaining the strength of the current necessary to 
overcome chemical affinity. Julius Thomsen investigated 
from a thermo-chemical standpoint the formation of a 
number of salts. From the chemical value of copper 
sulphate, obtained by Thomsen, can be calculated the 
voltage necessary to decompose the compoimd and give 
the metallic copper. 

However inadequate, thermo-chemical investigations 
have proven in the past, it is not unlikely that they may yet 
be fotmd of value in the future in more perfectly eluci- 
dating the doctrine of affinity of the elements. 



ELECTRO-CHEMICAL THEORIES. 

The first electro-chemical theories were enunciated in 
the early years of the nineteenth centiury. In 1801, 
two Englishmen, Nicholson and Carlisle, had succeeded 
in increasing the efficiency of the volta battery to such 
a degree that they obtained a current from it capable 
of decomposing water. 

In 1803, Berzelius and his student, Hisinger, published 
a paper in which they described the decomposition, 
by means of an electric current, of certain salts into their 
bases and acids. 

In 1800, Sir Himiphrey Davy began his investigations 
of the action of the current on chemical compounds, 



THE THEORIES OF CHEMISTRY. 155 

and in 1807, obtained a sufficiently strong current, from 
the volta pile to decompose caustic potash and caustic 
soda, and in 1808, the hydroxides of calcium and barium, 
obtaining their corresponding metals. These were valu- 
able discoveries in the judgment of Berzelius. 

In 1807, Davy announced the first "electro-chemical 
theory," in which he expressed the beHef that bodies hav- 
ing an affinity for one another were in different electrical 
states, and that chemical and electrical attractions 
depended upon the same cause, acting in the one case 
on particles and in the other on masses of matter. 

After these observations were made, it was but natural 
for chemists to make every effort to identify chemical 
affinity with electricity. 

In 1812, Berzelius brought forward the main points of 
his ''electro-chemical theory," page 90. He stated that 
a relation existed between chemical and electrical pro- 
cesses. Electric fire, Berzelius thought, was the same 
as fire originating from chemical combustion, because 
the electric spark will heat, melt and ignite all combus- 
tible substances and volatile metals. Prolonged action 
of the electric spark heats water to boiling and solids 
to redness. Carbon in a vacuum will be made to glow 
by a current from an electric battery and the same action 
is observed as carbon burning and oxidizing in the air. 
Combustion was, therefore, an electro-chemical phe- 
nomena, and all phenomena not satisfactorily explained 
by heat could be explained by means of electricity. 

Concerning the current obtained from the volta pile — 
Volta thought that metal contact was the cause of the 
current; while Davy contended that "electric condition 
increases in proportion to the affinity of the two metals 
acting. Temperature also influences electric condition." 



156 THE THEORIES OF CHEMISTRY. 

Assuming that substances were composed of atoms, 
Berzelius believed that these ultimate particles of matter 
possessed ** polarity" upon which the electric phenomena 
shown by the atoms was dependent. He believed that 
each atom was inherently "electric" and that this elec- 
tricity passed to two opposite poles — each atom, there- 
fore, possessing two kinds of electricity — the one pole 
being electro-positive and the other being electro-nega- 
tive. Later, Berzelius stated that the quantities of elec- 
tricity of the atoms of elements were different, so that 
elements were either electro-positive or electro-negative 
depending upon the preponderance of one kind of elec- 
tricity over the other in the atom. 

According to this idea of unipolarity or one-sidedness, 
an atom possessed two kinds of electricity, but if the pos- 
itive was in the greater degree the atom became known 
as an electro-positive atom, and if the negative was in the 
greater amoimt the atom was then known as an electro- 
negative atom. Berzelius, therefore, divided the elements 
into two groups: — electro-positive and electro-negative 
elements according to their prevailing electricities. He 
also extended this idea to embrace compounds. 

Berzelius' ''electro-chemical theory" was first published 
in Sweden in 1814, in France in 1819 and in Germany 
in 1820. It dominated chemical thought from 1820 
to 1840. 

Berzelius and Hisinger had conducted a current through 
a solution of sodium sulphate, contained in a V-shaped 
tube. The contents of both arms of the tube were tested 
after the current had been passed. With litmus, the con- 
tents of one arm gave the red o] acids (hence electro—), 
and the contents of the other arm gave the blue of bases 
(hence electro-}-). In other words, by the decomposition 



THE THEORIES OF CHEMISTRY. 157 

of sodium sulphate, by the electric current, the salt 
had been separated into its component parts Na2d.S03. 
Potassiimi sulphate, similarly, gave K2O.SO3. The me- 
tallic oxide in both instances, was the positive constitu- 
ent of the salt, whereas the oxide of the non-metal (SO3) 
was the negative constituent of the salt. These two 
electrified components, when combined, gave rise to 
the salt, neutral in its chemical reaction. Berzelius con- 
cluded, from these experiments, that salts consisted of 
two distinct parts (a basic and an acid portion) electric- 
ally different. In consequence of the above conclusion, 
he formulated his "duaHstic theory," page 89. 

Berzelius believed that when two bodies are acting 
electrically and fire appears, the current ceases. When 
two opposite electricities unite, the imion is accompanied 
with a spark; when substances have combined and are 
no longer electric (they being neutral) and are then sep- 
arated, the elementary atoms again take on their original 
electricity. Chemical union occurs because of the imion 
of different electricities, and in every chemical union there 
is an explanation for combustion. There is a neutrali- 
zation of opposite electricities, which neutralization pro- 
duces fire and heat, as well as light. Substances after 
combination, remain in the compound with an electric 
force greater than any force which would bring about a 
mechanical separation. Was this force inherent in the 
atoms or was it a property of electricity? This question 
troubled the great master. 

In the classification of elements and compounds accord- 
ing to their electricities: Oxygen and the other non- 
metals were placed in the electro-negative group. Hydro- 
gen and the metals were arranged according to the strength 
of their electrical affinities in the electro-positive group. 



158 THE THEORIES OF CHEMISTRY. 

Acid oxides were placed in the electro-negative group 
and basic oxides in the electro-positive division. Salts 
were also classified — those which united to form new 
compounds — ^to-day called "double salts," for example, 
Mg0.2Na2S04 and PtCl4.2KCl as electro-positive and 
electro-negative. 

How does electricity exist in these bodies? How can 
a substance be electro-negative or electro-positive? 
These were questions presenting themselves, not only 
to Berzelius, but to all others. Berzelius stated that 
substances were not only electric but they must be electric 
to their minutest particles — ^the atoms. This thought 
gave rise to the "theory or philosophy of electro-chem- 
istry — or the corpuscular theory." 

But specific *'unipolarity" did not serve to explain 
all of the phenomena, so that it was concluded that 
chemical union depended upon the intensity of polarity. 
The affinity of elements was an expression of their par- 
ticular electricity. 

These were, briefly, the views held by Berzelius. His 
"electro-chemical theory" differed from the "electro- 
chemical theory" of Davy. It was Davy's belief that 
the atom was electrically neutral and acquired either its 
positive or negative charge of electricity by contact with 
other atoms. 

From 1820, when Berzelius' idea began to be widely 
discussed, until 1840, he fought for his electro-chemical 
theory; upheld at first by Wohler and Liebig. But, 
when ideas and facts grew too numerous, particularly in 
the organic field, to make its further acceptance pos- 
sible, Berzelius modified and remodified it from time to 
time until in 1840, it received its death blow. 

The first criticism which Berzelius' electro-chemical 



THE THEORIES OP CHEMISTRY. 159 

theory received was, if chemical iinion is produced by the 
electrical attraction of oppositely charged atoms alone, 
then, when these atoms come together, their electric 
differences would disappear, and in consequence, the 
compound would fall apart; or as soon as the atoms 
separated, they would take up their respective elec- 
tricities, and would, therefore, immediately recombine; 
hence, compounds would alv/ays be in the state of un- 
stable equilibritim and complete decomposition would be 
almost impossible. 

A second objection to this theory was, if chemical imion 
depended only upon electrical attraction, then the prop- 
erties of the compounds must depend upon the nature 
of the charges existing in the atoms in the compounds; 
therefore, when it was foimd to be possible to substitute 
three atoms of chlorine for the three atoms of hydrogen 
in acetic acid, and fiurther that the properties of tri- 
chloracetic acid were almost identical with the properties 
of acetic acid, facts were being obtained which were 
opposed to Berzelius' theory. This was the chief objec- 
tion against this theory. 

Recall the recent work of J. J. Thomsen, page 102, who, 
having electrolyzed hydrogen, found some hydrogen at the 
negative pole and some at the positive pole. The impor- 
tant point, then, is that hydrogen is not always positive. 
The chlorine derivatives of methane were investigated 
and it was concluded that the chlorine in these com- 
pounds was charged with the same kind of electricity 
as that possessed by the hydrogen which the chlorine 
had replaced. If these observations are true then the 
chief objection to the theory of electro-chemistry of 
Berzelius is left without a foundation. 



160 THE THEORIES OF CHEMISTRY. 

Following the enunciation of the electro-chemical theory 
by Berzelius, the next important advance in electro- 
chemistry was made by Faraday. He showed the identity 
of electricity from different soiirces; and investigated 
the relation existing between the amotmt of a compound 
decomposed by the current, and the amount of the current 
used. He found that the two were proportional to one 
another and announced his "law of electro-chemical 
equivalents" in consequence. "The amount of chemical 
decomposition effected by the passage of the current is 
proportional to the amoimt of electricity which flows 
through the conductor." 

Faraday passed the same amount of current through a 
ntmiber of different metallic salt solutions. His conclu- 
sions were these: — "The amounts of the different elements 
which are separated by the same quantity of electricity 
bear the same relation to one another as the equivalents 
of these elements. The atoms of all imivalent elements 
carry exactly the same quantity of electricity — of bivalent 
elements twice as much, of trivalent three times, and so 
on. In a word, all atoms have either the same capacity 
for electricity, or a simple rational multiple of the capacity 
of the univalent atoms." 

Faraday is also the author of the system of nomencla- 
ture used to-day in electro-chemistry. 



At this point, a digression will be made in order to 
trace the development of ideas concerning "solution" 
and the connection of these ideas with the notions of 
electro-chemistry : 

It had been early observed, that to have chemical action 
one body must have motion, must be mobile, so that one 



THE THEORIES OF CHEMISTRY. 161 

reacting substance, at least, could present its polarity 
to the other. Heat is required to unite most gaseous 
bodies. If the particles are driven too far apart, how- 
ever, they will not unite because they have lost their 
power of electro-chemical activity. Union between solids 
does not often occur, but takes place when one of the 
solids is dissolved and more readily when both are brought 
into solution. An old chemical expression arose "Cor- 
poranon agunt nisi liquida." Liqmd bodies act on each 
other when surfaces exposed are large. 

Solutions: — First, a solution in the broadest sense of the 
word is a homogeneous mixture which cannot be separated 
into its component parts by simple mechanical means. 
Powders and emulsions which may be with very little 
expenditure of time and labor resolved into their con- 
stituents by lixivation, subsidation and the like are not 
solutions. A solution exists only when there is a complete 
and mutual interpenetration of the individual constituents, 
that is an interpenetration which extends down to the 
molecules themselves. 

Solutions are of three kinds, — ^gaseous, liqtiid and solid. 

Solution of Gases: — Gases mix in every proportion and 
interpenetrate one another completely, so that gas mix- 
tures are always gaseous solutions. The pressure P of a 
mixture of gases is equal to the simi of the partial pres- 
sures p', p'', etc., of the individual constituents. 

Solution of Gases and Liquids: — By absorption, gases 
unite with liquids to form liquid solutions. From Henry's 
law, it is inferred that the quantity of gas taken up by a 
liquid is proportional to the pressure imder which that 
gas exists. 

Solution of Liquids: — Liquid solutions may be arranged 
into two groups: first, those in which the constituents 



162 THE THEORIES OF CHEMISTRY. 

mix in every possible proportion; second, those in which 
the components are only partially soluble one in the 
other. Group I is illustrated by pairs of liquids such as 
water and alcohol, carbon bisulphide and alcohol. Group 
II, by water and aniHne, or water and acetic ether. It is 
altogether probable that there is no deep-seated difference 
between the two groups, because every pair of liquids 
has its critical point beyond which the pair of the one 
group may belong to the one above or to the one below. 

The properties of solutions of liquids in one another 
caimot, as a rule, be calctilated out beforehand, as in the 
case of gases, from the properties of their constituents. 
In mbdng liquids, there are always contractions, the 
volume of the mixture is not equal to the sum of the 
volumes of the components. Again, changes appear in 
color, in refraction, in specific heat, and the like, which 
changes do not often correspond with those calculated 
from the rules laid down for mixtures. 

Solution of Solids: — Solid solutions are produced when 
the substance to be dissolved penetrates into the pores 
of the solid, for example, iodine into starch. Solid solu- 
tions are often produced when liquid solutions congeal 
or when crystals separate from solutions. The color 
of crystals may also be included under this head of solid 
solutions. 



Dilute Solutions: — The behavior of dilute liquid solu- 
tions is of very great theoretical importance. In such 
solutions one of the components (the solvent) is, in 
comparison to the other (the dissolved substance) in 
very great excess. This example illustrates the point — 



THE THEORIES OF CHEMISTRY. 163 

If a layer of pure water is poured over a sugar solution, 
the two layers will gradually mix because the sugar must 
pass from points of a higher concentration to points of 
a lower concentration, that is, it wanders upward, and 
eventually there results a dilute sugar solution; but, if 
a semi-permeable diaphragm were placed between the 
layers of pure water and a sugar solution, the penetration 
of the sugar from the lower layer to the upper layer may 
be prevented. Such a diaphragm may be constructed in 
various ways. The most familiar method consists in 
saturating a porous plate with copper sulphate solution 
and then dipping it into a solution of potassium ferro- 
cyanide, there results in the pores of the plate a reddish- 
brown precipitate of copper ferrocyanide, which becomes 
the membrane, or diaphragm, preventing the passage of 
the dissolved sugar from one liquid into the other but 
permitting the passage of water. 

The sugar molecules, which still have the tendency to 
pass from their solution into water, that is, to diffuse into 
the water, are prevented from so doing by such a mem- 
brane and naturally exert upon it a pressure, which 
pressure has been called "osmotic pressure; " this has been 
measured by Pfeffer. A porous cup had formed in it a 
copper ferrocyanide membrane, and there was poured 
into it a sugar solution, the membrane hinders the out- 
ward passage of the sugar molectdes. The porous cup 
was then closed with a stopper carrying a manometer 
and introduced into water, the water presses inward, 
exerts a pressure on the interior of the cup upward indi- 
cated by the manometer, which pressure represents the 
osmotic pressure of the sugar solution. The osmotic 
pressure for a four per cent, sugar solution is greater than 
the pressure of a quarter of an atmosphere. 



164 THE THEOklES OF CHEMISTRY. 

Nemst has suggested certain principles which take 
into consideration the influence exerted by the nature of 
the solvent upon the osmotic pressure. If two solvents 
are used which are only slightly soluble in each other, 
the dissolved substance distributes itself in a constant 
ratio between the two solvents. Hence it was concluded 
that the osmotic pressure was independent of the natiure 
of the solvent. 

This principle, other physical chemists contend, is 
only of value in the form in which it has been given with 
this proviso; that there is an equal molecular weight of 
the dissolved substance in both solutions; if that be 
not the case then Nemst's principle must be changed. 

If the temperature remains constant, the proportion 
between the pressiu-e and the molecular weight will 
remain definite. Osmotic pressure increases with tem- 
perature. For all substances which are similarly dissolved, 
the increase of pressure per degree is the same as that 
observed for gases, so that the osmotic pressure formxila is 



-•(■+^.) 



Pf 1+0.0036651), 



in which P is the pressure at t for zero degrees or t = 273. 
The osmotic pressure of a sugar solution is just as great 
as the pressure which that sugar would exert if it coiild 
occur as a gas in the same space occupied by the 
solution. 

What has been said for sugar in solution applies uni- 
versally to all dilute solutions. 

The hypothesis of Avogadro can be applied to dilute 
solutions, — "solutions which at like temperatures have 
like osmotic pressures in equal volumes contain the same 
number of molecules of dissolved substances." Such 



THE THEORIES OF CHEMISTRY 165 

solutions shov/ equal elevation in boiling point, equal 
reduction or lowering in the freezing point, and the degree 
of elevation or the degree of lowering is dependent upon 
the niimber and not upon the nature of the molecules 
of the substance dissolved. 

A more complete exposition of the foregoing facts will 
be found in the "Zeitschrift fur physikalische Chemie," 1, 
481 (1887), in an article entitled the "Roll of Osmotic 
Pressure in the Analogy between Solutions and Gases," 
by van't Hoff. 

To show how Avogadro's hypothesis is confirmed by the 
deportment of solutions, van't Hoff in that article offers the 
following experiment made with a one per cent, sugar 
solution, that is, a solution of one gram of sugar in one 
himdred grams of water. It has, therefore, one gram 
of sugar in about 100.6 cc. of solution. If the osmotic 
pressure of this solution be compared with the tension 
of some gas, like hydrogen, which contains just as many 
molecules for the 100.6 cc, it is discovered that there is 
a most astonishing concordance in the result. Pfeffer 
found that such a solution at 0° gives a pressure of 0.649 
atmosphere. As hydrogen at one atmosphere weighs per 
litre 0.0896 gram and the preceding concentration gives 
0.0581 gram per litre, then the pressure for the hydrogen 
for similar concentration and at 0°, calciilated from the 
gas laws, equals 0.656 atmosphere, which compares 
favorably with the value 0.649 atmosphere obtained for 
the sugar. Investigations were similarly conducted at 
other temperatures and the conclusion was reached that 
the osmotic pressure directly deduced from a sugar solu- 
tion is at the same temperature or at like temperatures 
nearly equal to the tension of a gas, which contains just 
as many molecules as there are sugar molecules in the 
same volume of the solution. 



166 THE THEORIES OF CHEMISTRY. 

This is experimental work, comparing a sugar solution 
with a gas, hydrogen. Similar experiments were con- 
ducted with malic acid, tartaric acid, citric acid, mag- 
nesium malate, invert sugar and the results were very 
concordant. 

Among gases there are certain irregularities due to dis- 
sociation phenomena, so too, there are exceptions to these 
observations on the part of certain other substances which 
are dissolved and these exceptions are particularly notice- 
able among acids, bases and salts which show remarkable 
and, at times, very great variations. 

From this work — the step to the consideration of the 
action of the current of electricity upon dilute solutions, 
is but a natural one. 



It will be recalled that the power of the current to 
decompose chemical compounds had been made promi- 
nent by the work of Faraday. Electrolysis is the term 
applied by him to this phenomena. Some of the most 
interesting advances made in electro-chemistry have been 
along this line of inquiry. Upon immersing the two poles 
of a voltaic cell into water containing a small quantity of 
sulphinic acid, hydrogen will be liberated at the one pole, 
and oxygen will appear at the other. Davy conducted an 
experiment to prove that the hydrogen and the oxygen 
liberated had not come from the same molecule of water. 
The action of the current upon other chemical compounds 
has been briefly set forth. Various theories were sug- 
gested from time to time to account for the reactions 
taking place; the last of these is the "theory of dissocia- 
tion of substances dissolved in water,*' by Svante 
Arrhenius. 



THE THEORIES OF CHEMISTRY. 167 

Arrhenius, a Swede, was attracted by the relations 
existing between solutions and gases — the work of van't 
HofiE, and in the "Zeitschrift fiir physikalische Chemie," 
Vol. 1, 631 (1887), published an article (translated) upon 
the dissociation of solids in water, in which he attempts to 
prove that salts, acids and bases manifest a higher osmotic 
pressure, than they are entitled to if the hypothesis of 
Avogadro for dilute solutions is applicable to them. 
It must be admitted for such solutions that the water 
exerts an influence which tends, so to speak, to destroy 
the molecules; in other words, the molecules of salts, 
bases, and acids, it must be granted, are separated into 
fractions, and these parts no longer conduct themselves 
in the manner noticed with sugar dissolved in water. 
Arrhenius beheved that only those substances, which were 
decomposed into fractions upon solution in water con- 
ducted the current. Such substances he termed electro- 
lytes. In the case of electrolytes there is every reason to 
believe that the original substances have, by solution, 
been dissociated; that the moleciiles of these bodies con- 
ducting the current have been broken down in aqueous 
solution into parts which are positively and negatively 
charged; and that these parts behave like distinct inde- 
pendent molecules, which Arrhenius called ions, those 
having the positive charge were called kations, those having 
the negative charge, anions. 

If the kations migrate in one direction and the anions 
in the other a transportation of electricity occurs, or, 
in other words, there is produced an electric current, 
so that the direction of the ciurent is connected with the 
presence of ions traveling about in the solution. 

Naturally these groups in their ionic condition have 
properties very different from those which the atoms 



168 THE THEORIES OF CHEMISTRY. 

possess in their neutral state when they are free from 
electricity. Arrhenius sought to explain this — Dilute 
hydrochloric acid consists in addition to undissociated 
molecules of hydrochloric apid — positively charged hydro- 
gen and negatively charged chlorine ions. The hydrogen 
kations swimming about in the hydrochloric acid mani- 
fest, in consequence of their similar charge, no desire 
whatever to combine with one another and to escape as 
hydrogen gas, H2; on the contrary they tend to repel 
one another and it is only after they have lost their charge 
that it is possible for them to recombine. This happens 
when an electric current is sent through the liquid. In 
the passage of that current the hydrogen kations, which 
are present collect about the negative electrode, give 
up to that electrode their positive charge and pass back 
into electrically neutral hydrogen atoms which quickly 
combine by twos and escape in the form of hydrogen 
gas, H2. This also applies to the chlorine anions except- 
ing that they lose their negative charge at the positive 
electrode where chlorine appears. 

The ions H and CI, dissolved in water do not possess 
the same properties as the gases H2 and CI2. In the 
hydrochloric acid the dissociation is into H-}- and CI— ; 
the number of the negative chlorine ions is always equal 
to the nimiber of the positive hydrogen ions. Inasmuch 
as hydrochloric acid in its ordinary state is completely 
non-electric, and as consequently the sum of the dis- 
charges of all the ions that can be present in it are abso- 
lutely equal to zero, then the absolute value of the positive 
charge of a hydrogen kation must be as great as the nega- 
tive charge of a chlorine anion, so that the chlorine and 
hydrogen ions are not only chemically but they are also 
electrically equivalent to each other. If the current be 



THE THEORIES OF CHEMISTRY. 169 

sent through hydrochloric acid, for every atom of hydro- 
gen set free at one electrode there must be an atom of 
chlorine liberated at the other. If this were not the case, 
if more hydrogen was Hberated than the equivalent of 
chlorine then there would be in the liquid more posi- 
tive than negative electricity and that liquid wotdd have 
a negative charge, whereas in reality hydrochloric acid 
is completely neutral, electrically speaking. The same 
is true of hydrobromic acid and hydriodic acid and, 
according to this ion theory, an acid is an electrolyte 
consisting of hydrogen kations. A base is an electrolyte 
that contains hydroxyl anions. An acid gives as many 
hydrogen ions as it has basicity. The haloid acids are 
monobasic and they yield but one hydrogen ion; the 
dibasic acids dissociate into two hydrogen ions. Acids 
dissociate into hydrogen ions and a radical, for example, 
sulphuric acid dissociates into H ions (two of them) 
and into the (SO4) radical; the charge of the (SO4) 
anions is equal to two times (e) where (e) represents 
the charge of H. All that has been said about acids 
holds true of bases. 

Salts dissociate into metal kations and acid radical 
anions. The magnitude of the charge may be calculated. 
A silver ion is equivalent to a hydrogen ion, both have 
the same charge. By the passage of an ampere of elec- 
tricity, which is called a coulomb, through a silver salt 
solution Kohlrausch says that 0.001118 gram of silver 
wiU be separated, the quantity of silver divided by the 
atomic weight of silver will give the gram equivalence 
of silver having a charge of one coulomb, whence it fol- 
lows that the charge of one gram of silver or a gram 
equivalent of silver equals 96540 coulombs, and for a 
gram equivalent of an ion the charge (e) is equal to the 



170 THE THEORIES OF CHEMISTRY. 

number 96540 coiilombs; the absolute value of an electric 
charge for an ion, therefore, might equal 8 X 1020 coulombs. 
The ions can move and wander about, or migrate, 
as it is termed. They have a velocity of migration. 



The essentials of the various chemical theories have 
been presented. No study of chemistry is complete 
without a knowledge of these landmarks of chemical 
science. 



INDEX 



Acetic acid, 100, 101, 112-115, 117 
Acidifying principle — 
hydrogen, 92 
oxygen, 9, 18-20, 92 
Acid material, 9 
Acids — • 

basicity of, 93 

definition of, 89, 92, 93 

dissociation of, 169 

haloid, 92 

hydrogen theory of, 92-94 

inorganic, 92, 93 

neutralization of, 46, 48 

organic, 93 

oxy-, 92 

oxygen theory of, 19, 20, 92 

radicals of, 93, 94, 169 
iEtherin — 

radical, 95-97 

theory (see theory) 
Affinity, chemical — 

degrees of, 132, 133 

doctrine of (Bergmann), 45 

doctrine of (Berthollet) , 49 

doctrine of (later development), 131, 134-142 

early ideas of (Boyle) , 6 

unit of, 133 
Air— 

an element, 3 

composition of (Lavoisier), 13, 16, 17 

composition of (Mayow), 6 

composition of (Priestley), 13, 17 

composition of (Scheele), 14, 15 

elasticity of, 6 

fixed, 16, 17 
Alchemists, 4 
Alchemy, 4 
Alcohol — 

composition of , 95, 98, 106, 112, 113 

series, 106, 112, 113 

type, 106 
Alcohol and acetic acid, 114, 115 

. (171) 



172 INDEX. 



Aldehyde, 112, 113 

series, 112 
Alkali metals (see metals) 
Alkalies, decomposition of, 155 
Allotropy, 68, 69 
Amides, 107 
Ajnmonia, 24, 36 

composition of, 92, 95 

radical, 95 

type, 128-130, 132, 134 
Ampere, 51 

Amphid salts (see salts) 
Anaximander, 3 
Anhydrides, 107 

Antiphlogistic theory (see theory) 
Aristotle, 3, 4 

Aromatic compounds (see carbon compounds) 
Arrhenius, 166-170 
Artiad, 141 

Asymmetric carbon atom (see carbon atom) 
Asymmetry, molecular, 152, 153 
Atom — 

application of term, 83 

approximate size of, 35 

compound, 23 

conception of (Dalton), 22, 23, 28 

conception of (Newton), 22, 23 

defined, 81-83 

defined (Dalton), 28 

defined (Laurent), 75 

saturation capacity of, 129-142 

ultimate particle, 22, 28, 29, 83 
Atomic — chemical atomic theory (see theory) 
Atomic heat (see law of) 
Atomic refraction (see refraction) 
Atomic weights — 

defined, 75 

determination of, 31, 35, 36, 52-89 

importance of, 57 

methods used in determining (Berzelius), 55, 57, 58 

methods used in determining (Dalton), 31, 35-37 

methods used in determining (recent), 52-89 

standard for comparison of (Berzelius), 54-56, 63, 73 

standard for comparison of (Dalton), 35-37, 54-56, 63, 73 

standard for comparison of (discussion of), 58-61 

systems of, 58 

table of (Dalton's), 31 

table of (diadactic), 59, 60 

table of (international), 59, 60 
Atomicity, 135 



INDEX. 173 

Atoms — 

basicity of, 133 

characters for (Dalton), 29, 30 

electricity of, 156-158 

polarity of (see polarity) 

spacial arrangement of , 103, 105, 107-116, 152, 153 

symbols of (Berzelius), 33 

units of compounds, 132 

weight of, 22-24, 30, 31, 35-37, 52-89 
Auxiliary type, 128 

Avogadro, 40, 51, 73, 75, 76, 78, 82, 84, 85 
Avogadro's law or hypothesis (see law) 
Azote (see nitrogen) 

Bacon, Roger, 4 
Baeyer, 116 
Balance, 17 
Bases — 

dissociation of, 169 

neutralization of, 46-48 
Basicity of — 

acids, 93 

atoms, 133 
Becher, 6-8 
Benzamide, 97 
Benzene ring, 145, 151, 152 

formula of, 151, 152 
Benzoic acid, 97 

radical of, 96, 97 
Benzoic ether, 97 
Benzoyl — (proin, orthrin), 96, 97 
Benzoyl chloride, 97 
Bergmann, 9, 45 
Berthollet, 20, 24, 38, 39, 49, 92 
Berzelius, 33, 39, 51, 53-57, 62, 63, 65, 72-74, 76, 77, 89, 90-102, 

116-118, 123, 127, 150, 151, 154-160 
Bitter almond oil, 96, 97 
Bivalent, 134, 135 
Black, 9 
Boerhaave, 9 

Boiling point determinations, 144, 146 
Boullay, 95 

Boyle, Robert, 5-7, 9, 17 
Bruhl, 150 
Bunsen, 98 

Cacodyl compounds, 98 

Caesalpinus, 12 

Calces, 9, 14 

Calcination of metals (see metals) 



174 INDEX. 



Calx (oxide), 8, 10, 11, 14 
Cannizzaro, 77-83, 86 
Carbon — 

hydrogen compounds of, 3 1-34 

oxygen compounds of, 18, 23-25, 32, 34 

refractive quotient of, 149, 150 

saturated compounds of (open chain arrangement), 150, 151 

specific volume of, 144 

unsaturated compounds of (closed chain or benzene ring), 145, 
151, 152 
Carbon atom — 

asymmetric, 152, 153 

quadrivalency of , 131^ 133, 136-139, 145 
Cardan, 12 

Cavendish, 9, 14, 15, 17-19, 24 
Characteristic of organic compounds (Laurent), 107 
Charcoal, 18, 23-26 
Chemical — 

action, 6, 7 

affinity, 132, 135, 155-158 

combination (cause of), 6, 28, 29, 157 

compounds (see compoimds) 

decomposition (double), 44-47 

nomenclature, 20, 160 

notation (Berzelius), 33, 34 

notation (Dalton), 29, 30, 32-34 

type, 116 
"Chemical Method" (Laurent), Extracts from, 117-127 
Chemistry — 

antiphlogistic, 18-20 

inorganic, 77 

modern, 13, 19, 20 

organic, 77, 84, 94-97, 181, 184 

pWogistic, 7-19 

physical, 52, 63, 143-154 

quantitative era of, 1 7 

spacial (see theory of structural chemistry) 

thermo- (see thermo-chemistry) 

unitary science, 77 
Chlorine, 72, 92, 145 
Clausius, 84 
Clement, 25 

Co-efficient of refraction (see refraction) 
Combustible bodies, 7-9 
Combustion — 

Becher, views on, 6, 7 

Boyle, views on, 6 

correct explanation of, 16-19 

Hooke, views on, 6 

identified with electricity, 155, 157 



INDEX. 175 



Combustion — Continued 

Mayow, views on, 6 : 

Rey, views on, 9-13 

theory of — antiphlogistic (see theory) 

theory of — phlogistic (see theory of phlogiston) 
Compounds- 
action of electric current upon, 154-157 

constitution of , 7, 52, 90-97, 100, 103, 108-110, 142-144 

defined, 116 

electricity of, 156-158 

formulas of, 62, 66, 67, 76, 86 

heat sphere of, 105 

inorganic (constitution of), 91-94, 97, 103 

organic (classification of — Laiu"ent), 106, 107 

organic (constitution of ) , 91-94, 98, 119, 120, 126, 142, 144-147, 
150 

oxygen, important element of, 90, 100 

spacial or geometric arrangement of atoms — (see theory of 
structural chemistry) 

specific heats of, 73-75 

unitary ideas concerning, 127 ,128 

units of, 132 
Congress, Chemical (at Carlsruhe), 76-78 
Constant of organic compounds (Laurent), 107 
Constitution of compounds (see compounds) 
Copulas (see theory of) 
Copulated compounds, 100 
Corpuscula (elements), 6 

Corpuscular theory (see theory, electro-chemical) 
Coulomb, 169, 170 
Couper, 89, 131-135, 141 
Crookes, 88 
Cruickshanks, 25 
Crystalline form (see isomerism) 
Cyanogen, 95 

Dalton, 21-43, 47-51, 53-56, 62, 63, 90 

Davy, Humphrey, 20, 24, 37, 38, 40, 62, 63, 89, 92, 93, 121, 126,154 

155, 158, 166 
Decomposition (see chemical decomposition) 
Dephlogisticated (oxidized), 8 
Dephlogisticated air (oxygen), 13, 14, 17, 18 
Desormes, 25 

Diamond, compounds with oxygen, 25 
Dibasic, 93 

Didactic table of atomic weights, 59, 60 
Diffusion process for sugar, 163, 164 
Dimorphism, 70 
Disease, explanation of, 5 



176 INDEX. 

Dissociation (see theory of) 

Dobereiner, 86 

Dualism (see theory, dualistic) 

Dulong, 92 

Dulong and Petit, 64-70, 74, 143 

Dumas, 72-74, 76, 86, 89, 93, 95-101, 103, 116, 117, 127, 143 

Dyads, 140 

Earth— 

an element, 3, 5 

vitrifiable, an element, 7 

volatile, an element, 7 

volatile (cause of combustion), 7 
Electricity — 

action upon dilute solutions, 166-170 

character of, 155, 156 

from different sources. 160 

identified with chemical affinity, 155, 157, 158 

of atoms and compounds, 156-158 

voltaic, 154 
Electrode, 168, 169 
Electrolysis, 166-170 
Electrolytes, 167 
Electro-chemical— 

equivalents (see law of) 

theory (see theory) 
Electro-negative — 

compounds, 156-158 

elements, 100, 102, 156-158 
Electro-positive — 

compounds, 156-158 

elements, 100, 102, 156-158 
Elements — 

affinity of (Boyle), 6 

affinity — saturation capacity of (see theory of valency) 

as condensation products, 53, 86, 88 

as salt-producers, 93 

characters for (Berzelius), 33 

characters for (Dalton), 29, 30 

classification of (periodic), 86-89 

defined (Boyle), 5, 6 

defined (Lavoisier), 19 

electricity of, 100, 102, 156-158 

heat sphere of, 105 

refraction of, 149, 150 

specific volume of, 144-147 

valency of (see theory of valency) 
Elements of — 

Aristotle, 3, 4 

Boyle, 5, 6 



INDEX. 177 



Elements of — Continued. 

Dalton, 23, 26, 27 

Davy, 37 

Empedocles, 3 

Geber, 4 

Stahl, 7 
Elixir vitae, 5, 8 
Eller. 9 

Empedocles, 3, 4 
Equivalency, 135 
Equivalent weights (Wollaston), 63 

distinguished from equivalency, 135 
Eqmvalents — ■ 

defined, 75 

doctrine of (Bergmann), 45 

doctrine of (Dalton) — (see law of proportions) 

doctrine of (Richter), 45-48 

doctrine of (Wenzel), 44, 45 

electro-chemical, 73 

law of, 160 
Ether, 95, 98 
Ethyl radical, 98 
Ethylene — 

(astherin), 95-97, 106 

chloride, 95 

nucleus, 106, 112 

series, 106 

Fachsius, 12 

Faraday, 73, 97, 98, 160, 166 

Fire— 

an element, 3 

a prototype, 3 

cause of, 155, 157 

in state of action, 7 

nature of (Lavoisier), 19 

nature of (Scheele), 14, 15 

pure (phlogiston), 7 
Fixed air, 16, 17 
Fluorine, an element, 92 
Formulas — 

confusion in use of, 62, 66, 67, 76, 86 

structural (see theory of structural chemistry) 
Frankland, 89, 129-131 
French School of Chemists, 90, 101, 123 

Gas densities — 

standard of, 78 
Gas volumes (see law of) 



178 INDEX. 

Gases — 

law of (Boyle), 6 

solution of (see solution) 

table of relative density of, 79 
Gay-Lussac, 38-40, 50, 51, 63, 90, 92, 95, 98, 143 
Geber, 4 

Geometric isomerism (see theory of structural chemistry) 
Gerhardt, 72, 74-76, 83, 89, 100, 101, 108, 118, 127, 128, 132 
Gmelin, L., 86, 103-116, 152 
Graham, 33, 89, 93 
Grecian philosophers, 3 

Haloid salts (see salts) 
Halydes, 107 

"Hand-Book of Chemistry" (Gmelin), Extracts from, 104-115 
Heat- 
mechanical theory of, 84 

nature of (Berzelius), 155, 157 

nature of (Lavoisier), 19 

nature of (Scheele), 14, 15 

specific (see law of atomic heat) 

sphere of elements and compounds, 105 
Heat of formation (see thermo-chemistry) 
van Helmont, 5 
Heraclitus, 3 
Herschel, Sir John, 41 
Hexagon formula of benzene (see benzene) 
Higgins, William, 42, 4? 
Hisinger, 154, 156, 157 
van't Hoff, 108, 116, 153, 165-167 
Hoffman, 9, 97 
Holoformes, 107 
Homologous compounds, 144 
Hooke, 6, 7, 9 
Humboldt, 50, 51 
Hydrides, 107 
Hydrocarbons, 32, 144, 150 

isomorphous, 150 

normal, 144 
Hydrochloric acid, 26, 27, 92 

type, 128, 130, 134 
Hydrofluoric acid, 26, 27 
Hydrogen — 

acidifying principle, 92 

atomic weight of, 58, 62, 91 

(inflammable air), 14 

molecular weight of, 78-82 

specific volume of, 144 

standard for gas densities, 78 



INDEX. 179 

Hydrogen — Continued, 
theory of acids, 92-94 
type, 128, 129, 134 

latro-chemistry (medical chemistry) 5, 8 
latro-chemists, 5 
Index, refractive (see refraction) 
Inflammable air (hydrogen), 14 
Inorganic compounds (see compounds) 
International table of atomic weights, 59, 60 
Iodine, an element, 92 
Ions, 167-170 

anions, 167-170 

behavior of, 167, 168, 170 

character of, 167 

kations, 167-170 
Isomerism (isomorphism) — 

defined, 150, 151 

geometrical, early suggestions (Laurent), 103, 107, 108 

geometrical, extended (Gmelin), 108-116 

geometrical, later development, 144-147, 149-153 

kinds, 150, 151 

law of (see law of isomorphism) 
Isomorphism, (see isomerism) 

Kekule, 89, 129, 131-135, 141, 142, 151, 152 

Klaproth, 20 

Kohlrausch, 169 

Kolbe, 89, 129-132 

Kopp, H., 68, 71, 143-147, 150, 151 

Lactic acids, 152 
Landolt, 149, 150 

Laurent, 74, 75, 89, 99-101, 103-108, 111-113, 115-117, 127, 128 
Lavoisier, 9, 10, 13, 15-20, 25, 49, 89-92, 143 

Law of atomic heat or specific heat (Dulong and Petit), 65-69, 73-75, 
143 

aid in determining atomic weights, 65-69, 74, 75 

applied to compoimds (Neimiann), 73-75 

exceptions, 67-69 
Law of atoms (Cannizzaro), 81-82 
Law of Avogadro, 40, 51, 73, 75-86, 90, 143 

aid in determining atomic weights (see pages above) 

Cannizzaro's interpretation of, 77-86, 90 

probability of 84, 85 
Law of Boyle (gases), 6 
Law of electro-chemical equivalents (Faraday), 73, 160 j 

aid in determining atomic weights, 73 



180 INDEX. 

Law of Gay-Lussac (gas volumes), 38, 50, 51, 63, 65, 143 

aid in determining atomic weights, 63 
Law of Henry, 161 

Law of indestructibility of matter (Lavoisier), 19 
Law of isomorphism (Mitscherlich), 69-72, 143 

aid in determining atomic weights, 69-72 

limitations, 70-72 
Law of periodicity, 86-89 
Law of proportions, definite — 

(Dalton), 28, 43-46, 48, 49 

(Wenzel, Bergmann, Richter), 44-48 

(Berthollet, Proust), 49 
Law of proportions, multiple — 

(Dalton), 29, 37, 49 

(Richter), 49 
LeBel, 108, 116, 153 
Lemery, 14 
Libavius, 5, 12 

Liebig, 17, 89, 93, 94, 96-98, 101-103, 116, 158 
Light, refraction of, 148 

Linking, atom (see theory of structural chemistry) 
Longcamp, 119, 120, 126 
Lossen, 145, 146 
Lucca, Prof., 77 
Lully, Raymond, 4 

Magnus, Albertus, 4 
Malaguti, 101 
Mallet, 61 
Margraff, 9 
Marsh gas — 

(carburetted hydrogen), 31-34 

type, 129-131, 134 
Matter — 

a prototype, 3 

constitution of (chemical atomic theory — see theory) 

constitution of, 3-7 

law of the indestructibility of, 19 

properties of, 3-5 

ultimate particles of, 22, 23 
Maxwell, Clerk, 82, 84, 85 
Mayow, 6, 7, 9 
Mechanical type, 116 
Meinecke, 53 
Melsens, 101, 117 
Melting points, 144 
Mendelejeff, 57, 87 
Mercury — 

an element, 4 



INDEX. 181 



Mercury — Continued. 

constituent of animal and vegetable matter, 4, 5 

constituent of metals, 4 
Metals — 

alchemistic view of, 4 

alkali, 37, 92, 155 

calcination of, 9-13, 16 

composition of, 7, 18 

properties of, 4 

solution in acids (Lemery), 14 

solution in acids (Lavoisier), 18 

transmutation of, 4, 5 
Metamerism, 144, 151 
Methane (see miarsh gas) 
Methylene series, 115 
Meyer, Lothar, 57, 77, 78, 83, 86, 87 
Minerals — 

molecular refraction of, 150 

specific volume of, 147 
Mitscherlich, E., 64, 69, 70, 143 
Mixed types, 129 
Molecular — 

as3rmmetry, 152,153 

compositions, mode of expressing, 79, 80 

compositions, table of, 80 

refraction (see refraction) 

weight, defined (Laurent), 75 
Molecule — 

application of term, 83 

defined (Avogadro), 82 

defined (Cannizzaro) , 81, 82 

defined (Laurent), 75 

defined (Maxwell), 82 
Monads, 140 
Monobasic, 93 
Monobasic acid type, 107 
Monobromethane, 106 
Monochlorethane, 106 
Morley, Edward W., 58 
Morveau, Gu3^on, 25 
Muir, 68, 70 

Neilson and Pettersson, 67 
Nemst, 164 
Neumann, 9, 73-75 
Neutralization — 

series, 46-48 

table for bases and acids, 38 
Newlands, 86 



182 INDEX. 



Newton, 22, 23 
Nicholson and Carlisle, 154 
Nitric acid, 18 
Nitrogen (azote) — 

an element, 72 

oxygen compounds of, 20, 24, 34 

specific volume of , 144 
Nomenclature — 

chemical (Lavoisier), 20 

electro-chemical (Faraday), 160 
Notation, chemical (see chemical notation) 
Nuclei, 103-115 

primary, 104-107 

secondary, 104-107 
Nucleus theory (see theory) 
Nucleus type, 106 
Numbers, as a prototype, 3 

Odling, 86, 103 

defiant gas, 31-34 

Optical activity (see theory of structural chemistry) 

Organic chemistry (see chemistry) 

Osmotic pressure — 

formiila for the determination of, 164 

Nernst's principle of, 164 
Ostwald, 78 

Ousia ether, an element, 4 
Oxide (calx), 8 

Oxidized (dephlogisticated) , 8 
Oxy-acids (see acids) 
Oxygen — 

acidifying principle, 9, 18-20, 92 

atomic weight of, 58, 62 

center point of chemical system, 90 

compounds, 100 

constituent of water, 14 

(dephlogisticated air), 13, 14, 17-20 

discovery of, 9, 13, 16 

elastic substance, 16 

(fire air), 15 

oxygenizing principle, 18-20 

specific volume of, 144 

theory of acids, 92 

Paracelsus, 4, 5 

Pasteur, 116, 152, 153 

Pentabasic, 93 

Periodic arrangement of elements, 86-89 

Perissads, 141 



INDEX. 183 



Pettenkofer, 86 
Pettersson and Neilson, 67 
Pfeffer, 163, 165 
Philosophers, Grecian, 3 
Philosopher's stone, 5, 8 
Philosophy, pre-Socratic, 3 
Phlogisticated (reduced), 8 

air (ox3^gen), 13 
Phlogiston — 

constituent of water, 14 

described (Stahl), 7 

identified with coloring matter, 8 

identihed with flame, 8 

identified with hydrogen, 8, 14 

identified with light, 8 

identified with the principle of levity, 8 

theory of (see theory of phlogiston) 
Phlogistonists, 9 
Phosphoric acid, 18 

basicity of, 93 
Phosphorus, 18, 26 
Physical chemistry (see chemistry) 
Physico-chemical methods, 143-154 
Polarity of atoms — 

electric, 156-159 

intensity of, 158 

uni-, 156, 158 
Polybasic acids, 93 
Polymerism, 151 
Polymorphous, 70 
Pott, 9 

Priestley, 9, 13-17, 19, 24 
Primary material, 88 
Principle — 

acidifying, 9, 14, 18-20 

igneous (Becher), 7 

igneous (Stahl) — (see theory of phlogiston) 

of alkaHnity, 14 

of inflammability, 7, 14 

oxygenizing, 18, 19 
Properties of matter, 3-6 
Proportion numbers (Davy), 63 
Proportions, doctrine of (see law of) 
Proust, 49 

Prout, 53, 57, 62, 86, 88 
Prout's hypothesis. 53, 57, 62, 86, 88 
Pythagoras, 3 

Quadrivalent, 134, 135 
Quantitative era of chemistry, 17 



184 INDEX. 

Racemic acid, 152 
Radicals — 

assumption of existence of, 89-91 

compound, 94-103, 127, 134 

defined, 98 

oxygenated, 89-91, 93-97 

theory of (see theory) 

variation in, 103 
Ratio existing between the elements of water, 58, 62 
Ratio numbers (Gay-Lussac) , 63 
Reduced (phlogisticated), 8 
Refraction — 

atomic, 149, 150 

formula, 148, 149 

molecular, 147-150 

value of, 148, 150 • 
Regnault, 65, 101 

Relative weights of atoms (see atomic weights) 
Replaceable value of elements (see theory of valency) 
Residues (see theory of) 
Rey, Jean, 9-13, 17, 20 
Richter, J. B., 45-48 
Rose, H., 56 , 

Salt- 
as an element, 4 

constituent of animal and vegetable matter, 4, 5 

constituent of metals, 4 
Saltpeter, 6 
Salts— 

amphid (oxygen), 93, 94 

classification (according to electricities), 158 

classification (acid and neutral), 93, 94 

constitution of (Berzelius), 91, 94, 154-158 

constitution of (Gmelin), 108-111 

constitution of (Laurent), 117-127 

constitution of (Liebig), 94 

defined, 93 

dissociation of, 169 

formation of, 94 

haloid, 93, 94 
Saturation capacity (see theory of valency) 
Scaliger, 12 
Scheele, 9, 14, 15 
SebeUen, 57 
Sedgwick, Prof., 56 
Series, compounds in, 106, 107 
Smith, Angus, 40, 42 
Smith, Henry, 42 



INDEX. 185 

Solution — 

Avogadro's law applied to, 164-167 

definition of, 161 

dilute, 162-170 

electro-chemistry and, 160 

Henry's law of, 161 

kinds of, 161 

osmotic pressure of (see osmotic pressure) 

theory of (see theory of) 
Space chemistry (see theory of structural chemistry) 
Specific density, 148 
Specific gravity, 143 
Specific heat of — 

atoms (see law of atomic heat) 

compounds (Neumann), 73-75 
Specific volume — 

(Kopp), 143-147, 150 

(Thorpe and Lossen), 146, 147, 150 

aid in determining atomic and molecular weights, 144 

constant for elements, 144, 145 

value, 147, 150 
Spectroscopy, 68 
Stahl, 7-9, 17, 18 

Stereo-chemistry (see theory of structural chemistry) 
Stochiometry (Richter), 46-48 
Structiu-al chemistry (see theory of) 
Substitution (see theory) 
Sulphtu — 

as an element, 4, 18, 26 

constituent of animal and vegetable matter, 4, 5 

constituent of metals, 4 

specific volume of, 144 
S3rmbols (see chemical notation) 
System of the elements (periodic or natural), 86-89 

Table of atomic weights, 31, 59, 60 

Table of density of gaseous elements and compounds, 79 

Table of elements (periodic arrangement), 88 

Table of molecular compositions, 80 

Table of neutralization series (bases and acids), 48 

Tartaric acids, optical activity of, 152 

Tennant, Mr., 25 

Tetrabasic, 93 

Thales, 3 

Th^nard, 92 

Theory, astherin (Dumas), 89, 95-97 

decline of , 96 
Theory, antiphlogistic — modem theory of combustion — (Lavoisier), 
18-20 

adoption of, 19, 20 



186 INDEX. 

Theory, chemical atomic (Dalton), 21-43, 62, 89, 90 

authorship of, 42, 43 

introduction and acceptance of, 34, 35 

strengthened by Berzelius, 62 

verification of, 38-40 
Theory, dualistic (Berzelius), 89, 91, 92, 94, 96, 100-103, 116-120, 
124-127, 131, 157 

manifestations against, 92, 94, 100-102, 116 

overthrow of , 100, 103, 116 
Theory, electro-chemical (BerzeHus), 52, 53, 90, 100, 117-120, 124- 
126, 155-160 
manifestations against, 158, 159 
Theory, electro-chemical (Davy), 155, 158 
Theory, nucleus (Laurent), 89, 91, 103-117, 127 
Theory, radical — 

development of (Lavoisier), 89-91 

the earlier (Berzelius), 89-91 

the newer (Kolbe), 89, 129 

the older (Berzelius, Liebig, Wohler), 89, 94, 96-98, 102-104, 
127 
Theory, substitution (Dumas, Laurent), 89, 91, 99, 100-104, 115, 116, 

127 
Theory, type — 

the newer (Gerhardt, Laurent), 89, 116, 117, 127-134 
extension of (Kekul6), 89, 129, 131 

the older (Dumas), 89, 116, 127 
Theory of acids — 

hydrogen, 92-94 

oxygen, 19, 20, 92 
Theory of an ur-substance (Aristotle, Boyle), 3-5 
Theory of combustion (see theory of phlogiston and theory, antiphlo- 

dstic) 
Theory of copulas (Berzelius), 89, 100, 117, 124, 127, 131 
Theory of dissociation (Arrhenius), 166-170 
Theory of four elements (Empedocles) , 3, 4 
Theory of heat (Avogadro), 84 

Theory of linking of carbon atoms (see theory of structural chemis- 
try) 
Theory of phlogiston — 

beginnings of (Becher), 6, 7 

development of (Stahl), 7-20, 128 

development of ideas contrary to (Boerhaave, Hoffman, 
Lavoisier), 9, 13-17, 19, 20 

explanation of, 7, 8 

overthrow of, 13-16, 18-20 

value of, 7 
Theory of residues (Gerhardt), 89, 127 
Theory of solution, 52, 160-166 
Theory of stereo-chemistry (see theory of structural chemistry) 



INDEX. 187 

Theory of structural (stereo-, space, spacial) chemistry — 

beginning of (Laurent), 103-116 

extension of (GmeHn), 108-116 

later development of, 131, 133, 144, 145, 149, 151-153 
Theory of the alchemists, 4 
Theory of the iatro- chemists, 4, 5 
Theory of valency — saturation capacity— of the elements, 89,129-143 

(Kolbe, Frankland, Couper, Kekul6) 
Thermo-chemistry, doctrine of, 153,154 
Thomsen, Julius, 102, 154, 159 
Thomson, Thomas, 34, 37, 38, 53 
Thorpe, 145, 146 
Transmutation of metals, 4, 5 
Tribasic, 93 

Trichloracetic acid, 99-101, 116, 117 
Trimorphism, 70 
Trivalent, 134, 135 
Turner, 53, 62 
Types — 

chemical, 116 

mechanical, 116 

mixed, 129 

series of, 106 

theory of (see theory) 

Univalent, 134, 135 
Ur-substance — 

(Aristotle), 3 

(Boyle), 5 

Valency — 

development of (early), 129, 130 

development of (in connection with organic compoimds), 142, 
143 

development of (later), 130-142 

difficulties in applying to inorganic compounds, 142 

doctrine or theory of (see theory of valency) 

even, 140, 141 

expressed, 134-136, 142 

odd, 140, 141 

of the carbon atom (see carbon atom) 
Valentine, Basil, 4 
Vapour density determinations (Dumas), 72-74, 143 

aid in determining atomic weights, 72-74 

limitations, 73 
Villanovanus, Arnold, 4 
Vitriolic acid, 18 
Volta, 155 

Voltaic electricity (see electricity) 
Volume, specific (see specific volume) 



188 INDEX. 

Water — 

an element, 3 

as a prototype, 3 

composition and decomposition of, 14, 18, 24, 26, 36, 154 

constituent of all airs, 14 

formula for, 62, 63, 72, 76 

ratio of constituents of, 58, 62 

type, 128, 131, 134 
Wax, balls of (Gmelin), 110, 116 
Weber, 68 
Wenzel, 44-46 
Winkler, 61 
Wislicenus, 152, 153 
Wohler, 89, 96-98, 101-103, 158 

Wollaston, 63 v .. 

Wurtz, 41, 42 



SEP 



25 1918 



